Optical pickup apparatus and objective lens

ABSTRACT

An objective lens of an optical pickup apparatus converges a divergent light flux onto an information recording surface. The following conditional formula is satisfied: 
       |δ SA   1   /δU|·|δU|+|δSA   2   /δT|·|δT|≦0.07   λrms            where λ represents a wavelength of a light source, δSA 1 /δU represents a change of a spherical aberration for an object-to-image distance change δU (|δU|≦0.5 mm) and δSA 2 /δT represents a change of spherical aberration for a temperature change δT (|δT|≦30° C.), the object-to-image distance is a distance between the light source (a light emitting point) and the information recording surface.

BACKGROUND OF THE INVENTION

The present invention relates to an objective lens of an optical pickupdevice and to the optical pickup device, and in particular, to anobjective lens wherein magnification is finite and yet the temperaturecharacteristics are excellent for recording or reproduction for at leasttwo optical information recording media each having a transparent baseboard with a different thickness and to an optical pickup device.

With regard to a recording/reproducing optical system for opticalinformation recording media having a precision required for theconventional CD reproducing apparatus (incidentally, arecording/reproducing optical system or a recording/reproducingapparatus mentioned in the present specification includes a recordingoptical system, a reproducing optical system, a recording andreproducing optical system, and an apparatus employing the foregoing),an infinite conjugated optical system is disclosed in TOKKAISHO No.57-76512, and a finite conjugated optical system is disclosed inTOKKAISHO No. 61-56314. Further, for reducing occurrence of aberrationcaused by a temperature change in the case of using an objective lensmade of resins, those employing a coupling lens are disclosed inTOKKAIHEI No. 6-258573. However, lenses made of resin (plastic) are usedwidely for a recording/reproducing optical system, especially for itsobjective lens, because of the recent demand for low cost.

However, an objective lens made of resin materials has a problem thataberration caused by a change in a refractive index that is derived froma temperature change is greater than that of a lens made of glassmaterials. In general, a change of a refractive index in resin materialsis different from that of a refractive index in glass materials by tentimes or more. In this case, when a difference between a temperature ofthe standard design and a temperature in the environment used actuallyis represented by ΔT, aberration changed by this temperature differenceΔT is mainly tertiary spherical aberration. Let it be assumed that SArepresents the tertiary spherical aberration components of wave frontaberration expressed in an rms value, and a sign of SA is defined sothat SA is greater than zero when the spherical aberration is positive(over), while, SA is smaller than zero when the spherical aberration isnegative (under). Tertiary spherical aberration ΔSA (λrms) caused bytemperature change ΔT can be expressed in the following expression byusing numerical aperture NA of the objective lens on the opticalinformation recording medium side (on the image side), focal length f,image forming magnification m, proportion coefficient k and lightwavelength λ.

ΔSA/ΔT=k·f(1−m)⁴(NA)⁴/λ  (1)

Incidentally, when a lens made of a resin material has a positiverefracting power, if a temperature rises, its tertiary sphericalaberration turns out to be over. Namely, the coefficient k in theaforesaid expression takes a positive value. Further, when a single lensmade of a resin material is made to be an objective lens, thecoefficient k takes a greater positive value.

In the case of an objective lens used for a compact disc that is widelyused presently, it can be said that aberration caused by a temperaturechange in the environment used does not arrive at the problematic level,because NA is about 0.45. However, optical information recording mediaare now promoted to be of high density.

To be concrete, there has been developed DVD (storage capacity: 4.7 GB)which is in the size mostly the same as that of CD (storage capacity:640 MB) and has raised recording density, and it is now popularizedrapidly. For reproduction of DVD, it is normal to use a laser beam witha prescribed wavelength for which a wavelength of the light source is ina range of 635-660 nm. A divergent light flux emitted from a laser lightsource is made to be a collimated light flux by a collimator lensgenerally, and then, it enters an objective lens whose NA on the DVDside is 0.6 or more to be converged on an information recording surfacethrough a transparent base board of DVD.

In consideration of the foregoing from the viewpoint of wave frontaberration, when NA, for example, is increased from 0.45 to 0.6 in theexpression (1) above, wave front aberration Wrms is increased to(0.6/0.45)⁴=3.16 times.

Though it is considered to make focal length f to be small for thepurpose of keeping the wave front aberration small based on theexpression (1), in this case, it is difficult to make f to be smallerthan the present value, because it is actually necessary to secure adistance of focusing operation.

With the background stated above, there have been proposed various typesof objective lenses and optical pickup devices for conducting recordingor reproduction, by using a single light-converging optical system, fora plurality of optical information recording media each having atransparent base board with a different thickness. It is known that theuse of plastic lenses for the aforesaid objective lens and opticalpickup device is advantageous for lightening a load for an actuator inthe course of focusing and tracking, and for moving the objective lensrapidly, for making an optical pickup device to be light in weight, andfor lowering the cost. For example, there are known an objective lensmade of plastic and an optical pickup device employing the same whereina divergent light is made to enter the objective lens for recording orreproducing of CD for restraining occurrence of spherical aberrationcaused by a thickness difference between transparent base boards, byutilizing that a diameter of a spot necessary for recording orreproducing for DVD (thickness of the transparent base board is 0.6 mm)and CD (thickness of the transparent base board is 1.2 mm) each havingdifferent recording density for information, is different each other anda necessary numerical aperture of the objective lens on the image sideis different.

In the optical pickup device of this type, if an objective lens is madeto be the finite conjugated type objective lens which is suitable for adivergent light flux from a light source to enter and an optical pickupdevice is made to be one employing that objective lens, for bothrecording or reproducing of DVD and recording or reproducing of CD,there are obtained merits that the optical pickup device can be madesmall and compact totally and a collimator lens to make a divergentlight flux from a light source to be unnecessary. However, an objectivelens which is made of plastic and satisfies various performancesnecessary for an optical pickup device, and an optical pickup deviceemploying such objective lens made of plastic are not on practical use,and studies for them have not made yet.

On the other hand, in the case of a lens system using a conventionalobjective lens made of resin materials, there has been generatedaberration that is proportional to the fourth power of numericalaperture NA of the objective lens on the image side, and is caused byrefractive index change Δn of resin material derived from a temperaturechange, and this aberration has made it difficult to realize anobjective lens and an optical pickup device both having sufficientoptical performances.

With the aforesaid background, the inventors of the invention repeatedtrials and errors for realizing the objective lens and the opticalpickup device stated above, and found out that an improvement oftemperature characteristics of an objective lens is important for therealization. To be more concrete, they found out that the realizationcan be carried out by an objective lens and an optical pickup device,wherein there is provided a diffraction construction which makesspherical aberration for temperature changes to be satisfactory, on atleast a peripheral area on at least one surface of the objective lens.

A first object of the invention is to provide a practical objective lensand an optical pickup device, wherein a divergent light emitted from alight source enters the objective lens, and sufficient properties fortemperature changes in ambient conditions used are satisfied. Further,the first object of the invention is to provide a practical objectivelens and an optical pickup device, wherein a divergent light emittedfrom a light source enters the objective lens, for a plurality ofoptical information recording media each having a transparent base boardwith a different thickness, and sufficient properties for temperaturechanges in ambient conditions used are satisfied, while making recordingor reproducing for each information to be possible.

Further, the present invention relates to an objective lens and anoptical pickup apparatus having a good temperature characteristics and awide allowable range for a wavelength change of an light source.

An information recording surface of an optical information recordingmedium such as CD and DVD is usually protected by a transparent baseboard having a thickness stipulated by a standard. For conductingrecording and reproducing for the optical information recording media,there is used an objective lens that is corrected in terms of sphericalaberration for the transparent base board having that thickness. As anobjective lens for recording and reproducing for these opticalinformation recording media, various types of objective lenses are nowstudied, and TOKKAIHEI No. 6-258573, for example, discloses an objectivelens of a refraction type wherein both sides thereof are asphericsurfaces. On this objective lens, there is introduced an asphericsurface to correct aberration of an optical system.

FIG. 52 is a diagram showing how residual aberration (sphericalaberration) is generated when a thickness of the transparent base boardis changed. When the spherical aberration is worsened, a diameter of alight spot formed on an information surface of an optical informationrecording medium is changed from the desired diameter. The desired spotdiameter (range of 1/e² of peak intensity), in this case, isapproximated to Spot diameter (μm)=0.831×λ/NA, when the numericalaperture of the objective lens is represented by NA and a wavelength ofthe light source is represented by λ (μm). Therefore, furthertechnologies are needed for securing interchangeability of opticalinformation recording media each having a different thickness of thetransparent base board.

TOKKAI No. 2000-81566 discloses technologies wherein sphericalaberration for the specific transparent base board thickness iscorrected in the wavelength used for CD or DVD, when a diffractionsurface is united solidly with an aspheric surface of an objective lens.In this objective lens, over spherical aberration of base asphericsurface in a refraction system is corrected by under sphericalaberration generated on the diffractive section. In this case, thediffractive section has a function to correct spherical aberrationtoward the under side in CD having a thick transparent base board,because the diffractive section has power that is proportional to thewavelength. Therefore, if power allocation for the refraction sectionand the diffractive section is properly selected, it is possible tocorrect spherical aberration in the transparent base board thickness of0.6 mm for light source wavelength 650 nm in the case of using DVD andspherical aberration in the transparent base board thickness of 1.2 mmfor light source wavelength 780 nm in the case of using CD. Further,TOKKAIHEI No. 11-274646 discloses an example wherein there is provided adiffraction surface which corrects fluctuations of a focus positioncaused by a refractive index change resulting from a temperature changeof a plastic lens.

In these objective lenses, there is a tendency that a change ofspherical aberration caused by temperature changes is increased as thereare advanced a movement toward the finite of an optical pickup device, amovement toward a short wavelength and a movement toward high NA, forrecording and reproducing for high density information. Amount of changeδSA₃ of 3^(rd) order component of spherical aberration caused bytemperature changes is expressed by the following expression, when NArepresents a numerical aperture of an objective lens on the image side,f represents a focal length, represents an image forming magnificationand λ represents a wavelength of a laser light source.

(δSA₃/δT)∝f·(1−m)⁴·NA⁴/λ  (116)

Therefore, there is a tendency that temperature characteristics aredeteriorated more as a movement toward an objective lens for high NA anda movement toward the finite of the objective lens are advanced, or as amovement toward a short wavelength of a laser light source is advanced.Error characteristics (conventional Example 1) in the case of designingon a conventional refracting interface are shown in “Table 14”.Incidentally, from now on (including lens data of the table), the powermultiplier of 10 (for example, 2.5×10⁻³) is shown by the use of E (forexample, 2.5×E-3).

TABLE 14 Minimum δSA₃ (λ rms) pitch at DVD in δSA₃ (λ rms) of ring-Transparent temperature at DVD in shaped base board change wavelengthchange diffractive f (mm) λ thickness (δT = +30° C., (δλ = zone DVD m(nm) NA (mm) dn/dT(/° C.) δλ = +6 nm) +10 nm) Type of objective lens(μm) Conventional 3.0 0 785 0.50 1.2 −1.20E−04 +0.011(CD) +0.000 (CD)Refractive surface only — Example 1 Conventional 3.0 −1/7.0 650 0.60 0.6−1.20E−04 +0.098 +0.008 Refractive surface only — Example 2 Conventional3.05 −1/6 650 0.60 0.6 −1.20E−04 −0.002 +0.076 Diffractive surface 3Example 3 δSA₃ (λ rms) at δSA₃ (λ DVD in rms) at temper- DVD in aturewave- Minimum change length pitch of CD Transparent (δT = changering-shaped DVD spot base board +30° C., (δλ = Type of diffractive spotdiam- f (mm) thickness δλ = +10 objective zone diameter eter DVD m λ(nm) NA (mm) dn/dT(/° C.) +6 nm) nm) lens (μm) (μm) (μm) Example 1 3.0−1/7.0 650/780 0.60/0.45 0.6/1.2 −5.80E−06 +0.002 +0.007 Refractive —0.903 1.420 surface only Example 2 3.0 −1/7.0 650/780 0.60/0.45 0.6/1.2−1.20E−04, +0.027 +0.005 Refractive — 0.898 1.414 +0.8E−06 surface onlyExample 3 3.0 ∞ 660/790 0.65/0.45 0.6/1.2 −5.70E−06 +0.009 +0.008Diffractive 14 0.846 1.487 surface Example 4 3.0 ∞ 660/790 0.65/0.500.6/1.2 −1.20E−04, +0.019 +0.032 Diffractive  9 0.851 1.265 +7.4E−06surface Example 5 3.0 −1/7.0 650/780 0.60/0.45 0.6/1.2 −1.20E−04, −0.004−0.012 Diffractive 10 9.004 1.359 +0.8E−06 surface Example 6 3.0 −1/10.0650/780 0.60/0.45 0.6/1.2 −5.80E−06 +0.002 −0.001 Diffractive  8 8.9001.430 surface

For the problems mentioned above, there is considered a method toimprove temperature characteristics by employing diffraction, as shownin the prior art. However, when trying to improve temperaturecharacteristics by employing diffraction, following two troubles arecaused. First one of these troubles is that an objective lens turns outto be weak for wavelength characteristics. The direction in whichspherical aberration is generated by temperature changes on a refractionsection is originally different from that on a diffractive section, andwhen trying to improve temperature characteristics more, sphericalaberration generated on the refraction section alone is canceled bystrengthening effectiveness of the diffractive section relatively, butin the case of wavelength changes which are not followed by temperaturechanges, the aforesaid spherical aberration remains as residualaberration without being canceled, which is the reason why the objectivelens turns out to be weak for wavelength characteristics.

The second trouble is that when trying to make the effectiveness ofdiffraction to be great, diffraction pitch becomes small and diffractionefficiency is lowered. There is a tendency, in particular, that a pitchbecomes smaller as the position corresponding to the pitch moves in thedirection toward the periphery of the objective lens. In the case ofConventional Example 2 in “Table 14” wherein temperature characteristicshave been corrected thoroughly, a minimum pitch of the ring-shapeddiffractive zone is 3 μm and diffraction efficiency is lowered to about80% on the ring-shaped diffractive zone.

The invention is to solve the aforesaid problems, and the second objectis to provide an objective lens which makes it possible to conductrecording and reproducing for optical information recording media eachhaving a different transparent base board thickness such as DVD system(DVD-ROM and DVD+RAM) and CD system (CD-ROM and CD+RW) and an opticalpickup device, while securing excellent temperature characteristics.

SUMMARY OF THE INVENTION

Firstly, the structure to achieve the first object is explained.

When a diffractive section is provided on an objective lens, it ispossible to divide into a refracting power of diffraction basic asphericsurface and a diffracting power of the diffractive section, even in thecase of a single lens, and a degree of freedom in design is increased,compared with an occasion to construct a lens only with refraction. Ifthis power allocation between the refracting power and the diffractingpower is carried out properly, temperature characteristics can becorrected. Now, the correction of temperature characteristics in thecase of introducing a plastic objective lens in a finite optical systemwill be explained.

When ∂SA/∂T represents a change in an amount of tertiary sphericalaberration for temperature changes of a spherical-aberration-correctedpositive lens made of resin such as a single objective lens with anaspheric surface having no diffraction pattern that is commonly used forrecording and reproducing of optical information recording media, thechange is expressed by the following expression.

∂SA/∂T=(∂SA/∂n)·(∂n/∂T)+(∂SA/∂n)·(∂n/∂λ)·(∂λ/∂T)=(∂SA/∂n){(∂n/∂T)+(∂n/∂λ)·(∂λ/∂T)}  (4)

In this case, (∂n/∂T)<0 and (∂n/∂λ)<0 hold for resin materials.(∂n/∂T)=0 and (∂n/∂λ)<0 hold for glass materials. (∂n/∂T)>0 holds for asemiconductor laser and (∂λ/∂T)=0 holds for an SHG laser, a solid statelaser and a gas laser.

Incidentally, though (∂n/∂T) for glass materials and (∂λ/∂T) for an SHGlaser, a solid state laser and a gas laser are made to be zero, thesevalues are not zero to be exact. However, they are thought to be zeropractically in the field of the invention, and thereby, the explanationcan be simplified. Therefore, the explanation is forwarded under theassumption that these values are zero.

Now, when a light source is represented by an SHG laser, a solid statelaser or a gas laser, and (∂λ/∂T)=0 holds, the following expressionholds.

∂SA/∂T=(∂SA/∂n)·(∂n/∂T)  (5)

If this lens is made of glass, (∂n/∂T)=0 holds, and if therefore,∂SA/∂T=0 holds. On the other hand, if the lens is made of resin,(∂n/∂T)<0 holds, and (∂SA/∂n)<0 holds, because ∂SA/∂T>0 holds for thelens of this kind. Further, (∂λ/∂T)>0 holds when a light source isrepresented by a semiconductor laser.

In this case, even when the lens is made of glass, the followingexpression holds,

∂SA/∂T=(∂SA/∂n)·(∂n/∂λ)·(∂λ/∂T)  (6)

and ∂SA/∂T>0 holds because of (∂n/∂λ)<0 and (∂SA/∂n)<0.

When a wavelength of incident light turns out to be shorter irrespectiveof glass materials and resin materials, an absolute value of (∂n/∂λ)turns out to be greater. When using a semiconductor laser with a shortwavelength, therefore, it is necessary to pay attention to temperaturechanges for spherical aberration, even for glass materials.

On the other hand, when an amount of a change of tertiary sphericalaberration for temperature changes is formulated in terms of ∂SA/∂T,with respect to a resin aspherical single lens having a diffractionpattern, the following is obtained. In this case, it is necessary totake in both characteristics of the refracting power and characteristicsof the diffracting power. When R is suffixed to amount of change ∂SA ofa spherical aberration amount to which a refracting lens sectioncontributes, and D is suffixed to amount of change ∂SA of a sphericalaberration amount to which a diffracting power contributes forindicating, ∂SA/∂T can be expressed as follows.

∂SA/∂T=(∂SA _(R) /∂n)·(∂n/∂T)+(∂SA _(R) /∂n)·(∂n/∂λ)·(∂λ/∂T)+(∂SA_(D)/∂λ)·(∂λ/∂T)  (7)

In this case, when a light source is represented by an SHG laser, asolid state laser or a gas laser, and when (∂λ/∂T)=0 holds, thefollowing expression holds.

∂SA/∂T=(∂SA _(R) /∂n)·(∂n/∂T)  (8)

In the case of a glass lens, in this case, (∂n/∂T)=0 naturally holds,and ∂SA/∂T=0 holds independently of a value of (∂SA_(R)/∂n). In the caseof a resin lens, on the other hand, (∂n/∂T)<0 holds, and if(∂SA_(R)/∂n)=0 holds, ∂SA/∂T=0 can hold.

In the invention, therefore, a diffracting power is introduced to aresin aspherical single lens, so that (∂SA_(R/∂n)=)0 may hold withrespect to a refracting power. However, in the case of a refractingpower alone, spherical aberration remains, but the use of a diffractingpower makes it possible to correct spherical aberration of an opticalinformation recording medium on one side.

On the other hand, in the case of a light source represented by asemiconductor laser, (∂λ/∂T)>0 holds, and in the case of an objectivelens having characteristics of the aforesaid (∂SA_(R)/∂n)=0, thefollowing expression is obtained from the aforesaid expression (7).

∂SA/∂T=(∂SA _(D)/∂λ)·(∂λ/∂T)  (9)

However, (∂SA_(D)/∂a)≠0 usually holds, and it is understood that anamount of tertiary spherical aberration is changed by temperature.

Further, the expression (7) stated above can be deformed to thefollowing expression.

∂SA/∂T=(∂SA _(R) /∂n)·{(∂n/∂T)·(∂n/∂λ)·(∂λ/∂T)}+(∂SA_(D)/∂λ)·(∂λ/∂T)  (10)

In the case of a resin lens, in this case, (∂SA/∂T)<0 holds, a lightsource is represented by a semiconductor laser, and (∂λ/∂T)>0 holds.Therefore, the following expression is obtained.

(∂n/∂T)+(∂n/∂λ)·(∂λ/∂T)<0  (11)

When (∂SA_(R)/∂n)<0 holds as an assumption, the first term of expression(10) turns out to be a positive value from expression (11). To make∂SA/∂T=0 to hold, the second term needs to take a negative value underthe condition of (∂SA_(D)/∂λ)<0, because of (∂n/∂T)>0.

In the resin aspherical single lens having a diffracting power with thecharacteristics stated above ∂SA/∂T>0 holds because (∂SA_(R)/∂n)<0 and(∂n/∂T)<0 hold in the aforesaid expression (8), in the case of(∂λ/∂T)=0.

Spherical aberration ∂SA/∂λ in the case where a temperature is constantand a wavelength only changes can be expressed by the followingexpression.

∂SA/∂λ=(∂SA _(R) /∂n)·(∂n/∂λ)+(∂SA _(D)/∂λ)  (12)

Though the first term is positive and the second term is negative, thediffracting power mainly contributes greatly to chromatic aberration ofan aspherical single lens having a diffracting power as is known widely,thus, a sign of ∂SA/∂λ is determined by the second term of the aboveexpression (12), and ∂SA/∂λ<0 generally holds.

Namely, in the resin single lens into which a diffracting power isintroduced, it is possible to make ∂SA/∂T to hold even in the case of alight source represented by a semiconductor laser, by making∂SA_(R)/∂T>0 and ∂SA_(D)/δλ<0 to hold.

When (∂SA_(R)/∂n)>0 holds, on the contrary, it is possible to make∂SA/∂T to hold even in the case of a light source represented by asemiconductor laser, by making ∂SA_(R)/δT<0 and ∂SA_(D)/∂λ>0 to hold,though calculation is omitted here.

Namely, it is needed that a sign of ∂SA_(R)/∂T is opposite to that of∂SA_(D)/∂λ. In this case, the relationship of ∂SA_(R)/∂T·∂SA_(D)/∂λholds. The invention makes it possible to provide an objective lenswherein sufficient functions can be secured even for changes of ambienttemperatures used. In this case, when (∂SA/∂T) is made to be greaterthan zero, the characteristic of the objective lens is closer to that ofa resin aspherical single lens having no diffracting power, and thereby,a load of diffracting power is less, which is preferable. The inventionmakes it possible to provide an objective lens wherein sufficientfunctions can be secured even for changes of ambient temperatures used.

The objective lens having the structure stated above makes it possibleto correct spherical aberration and temperature for an opticalinformation recording medium on one side. Further, to conductrecording/reproducing of an optical information recording medium on theother side, optical surface areas which can divide a light flux enteringthe objective lens into some areas are formed on at least one side ofthe objective lens. Then, a certain light flux in an intermediatesection of the divided light flux is made to be a spherical aberrationdesign corresponding to a transparent base board thickness of the otherdisc. Satisfactory allocation of these divided light fluxes makes itpossible to correct spherical aberration and temperature of an opticalinformation recording medium on one side and to correct sphericalaberration of an optical information recording medium on the other side.

(1) The optical pickup device described in (1) having therein a lightsource and a light-converging optical system including an objective lensfor converging a light flux emitted from the light source on aninformation recording surface of an optical information recordingmedium, and is capable of conducting recording and/or reproducing ofinformation for a first optical information recording medium in which athickness of a transparent base board is t₁ and for a second opticalinformation recording medium in which a thickness of a transparent baseboard is t₂ (t₁<t₂) wherein the objective lens is a plastic lens, adivergent light flux emitted from the light source enters the objectivelens when recording or reproducing information for the first opticalinformation recording medium and when recording or reproducinginformation for the second optical information recording medium, and thefollowing conditional expression is satisfied when λ represents awavelength of the light source, δSA₁/δU represents a change of sphericalaberration for object image distance change δU (|δU|≦0.5 mm) and δSA₂/δTrepresents a change of spherical aberration for temperature change δT(|δT|≦30° C.).

|δSA ₁ /δU|·|δU|+|δSA ₂ /δT|·|δT|≦0.07λrms  (14)

In the optical pickup device described in (1), when the sum total of|δSA₁/δU|·|δU| and |δSA₂/δT|·|δT| is looked and it is made to be notmore than 0.07 λrms by providing the diffractive structure on theobjective lens, for example, it is possible to conduct properlyrecording or reproducing of information for two optical informationrecording media even under the condition that a divergent light fluxwith a single light source wavelength enters the objective lens, and itis possible to omit a collimator lens for forming a collimated lightflux that enters the objective lens, to attain cost reduction, and tomake the structure of the optical pickup device to be compact.

Incidentally, the word “object-to-image distance” means a distancebetween a light source (a light emitting point) and an informationrecording surface of an optical information recording medium.

(2) In the optical pickup device described in (2), at least one surfaceof the objective lens is provided with a diffractive structure on atleast a peripheral area in an effective diameter, and it is possible toconduct recording or reproducing of information for two optimalinformation recording media properly even under the condition that adivergent light flux enters the objective lens, because the followingconditional expression is satisfied when δSA1/δT represents a change ofspherical aberration for temperature change δT in a light flux which haspassed the diffractive structure on the peripheral area among lightfluxes emitted from the light source.

|δSA ₁ /δT|≦0.002λrms/° C.  (15)

(3) In the optical pickup device described in (3), it is possible toconduct recording or reproducing of information for two opticalinformation recording media properly even under the condition that adivergent light flux enters the objective lens, because the followingconditional expression is satisfied when δSA1/δT represents a change ofspherical aberration for temperature change δT in a light flux which haspassed the diffractive structure on the peripheral area among lightfluxes emitted from the light source.

|δSA ₁ /δT|≦0.0005λrms/° C.  (16)

(4) In the optical pickup device described in (4), the diffractivestructure on the peripheral area of the objective lens is a ring-shapeddiffractive zone, and with regard to a light flux passing through thediffractive structure of the peripheral area of the objective lens amonglight fluxes emitted from the light source, an average pitch P out ofthe ring-shaped diffractive zone satisfies the following expression.

2.00×10⁻⁴ ≦Pout/(|n|·f)≦3.00×10⁻²  (17)

(5) In the optical pickup device described in (5), an average pitch Pout of the ring-shaped diffractive zone satisfies the followingexpression.

1.00×10⁻³ Pout/(|n|·f)≦3.00×10⁻³  (18)

(6) In the optical pickup device described in (6), an average pitch Pout of the ring-shaped diffractive zone satisfies the followingexpression.

3.00×10⁻³ ≦Pout/(|n|·f)≦8.00×10⁻³  (19)

(7) In the optical pickup device described in (7), the optical surfaceof the objective lens is composed of three or more kinds of opticalsurface areas arranged in the direction perpendicular to an opticalaxis, and when the three or more kinds of optical surface areas arerepresented by an optical surface area closer to the optical axis, anintermediate optical surface area and an optical surface area closer tothe outside, all arranged in this order from the optical axis side, theoptical surface area closer to the outside is the area on the peripheralside stated above.(8) In the optical pickup device described in (8), spherical aberrationis discontinuous in at least one of a boundary between the opticalsurface area closer to the optical axis and the intermediate opticalsurface and a boundary between the intermediate optical surface area andthe optical surface area closer to the outside.(9) In the optical pickup device described in (9), a diffractive sectionwhere ring-shaped diffractive zones are formed is formed on the opticalsurface area closer to the optical axis, and average pitch P in of thering-shaped diffractive zones satisfies the following expression, whenn-th order light represents a diffracted light with a maximum amount oflight generated by the diffractive structure from a light flux passingthrough the diffractive structure on the optical surface area closer tothe light source among light fluxes emitted from the light source, and frepresents a focal length of the objective lens.

3.00×10⁻³ ≦Pin/(|n|·f)≦8.0×10⁻²  (20)

(10) In the optical pickup device described in (10), the optical surfacearea closer to the outside has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium stated above.(11) In the optical pickup device described in (11), when recording orreproducing information for the first optical information recordingmedium, spherical aberration of the light flux passing through theintermediate optical surface area is made to be discontinuous and to beflare component, for spherical aberration of the light flux passingthrough the optical surface area closer to the outside, while whenrecording or reproducing information for the second optical informationrecording medium, the light flux passing through the intermediateoptical surface area is used.

Incidentally, the flare component is one wherein an amount of sphericalaberration is given to the light flux passing through the intermediateoptical surface area, so that the light flux may be in the non-imageforming state at a focused position of a regular optical informationrecording medium, and the greater amount of spherical aberration ispreferable. Further, the greater amount of a difference of steps at aposition of a boundary between optical surfaces is preferable.

(12) In the optical pickup device described in (12), the intermediateoptical surface area has a function to correct spherical aberration forthickness t (t₁<t<t₂) of a transparent base board.(13) In the optical pickup device described in (13), when recording orreproducing information for the first optical information recordingmedium, a light flux passing mainly through the optical surface areacloser to the optical axis and the optical surface area closer to theoutside is used, while when recording or reproducing information for thesecond optical information recording medium, a light flux passing mainlythrough the optical surface area closer to the optical axis and theintermediate optical surface area is used.(14) In the optical pickup device described in (14), when recording orreproducing information for the second optical information recordingmedium, the following expressions are satisfied under the assumptionthat the intermediate optical surface area is formed within a range fromthe shortest distance from an optical axis NAH mm to NAL mm when anecessary numerical aperture is represented by NA₂ and a focal length ofthe objective lens is represented by f₂.

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (21)

(NA ₂−0.20)f ₂ ≦NAL≦(NA ₂−0.04)f ₂  (22)

(15) In the optical pickup device described in (15), when recording orreproducing information for the first optical information recordingmedium, a light flux passing through the intermediate optical surfacearea is made to have under spherical aberration.(16) In the optical pickup device described in (16), the optical surfacearea closer to the optical axis has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium.(17) In the optical pickup device described in (17), the optical surfacearea closer to the optical axis has a function to correct temperaturecharacteristics when recording or reproducing information for the firstoptical information recording medium.(18) In the optical pickup device described in (18), the optical surfaceof the objective lens is composed of two or more kinds of opticalsurface areas arranged in the direction perpendicular to an opticalaxis, and when the two kinds of optical surface areas are represented byan optical surface area closer to the optical axis and an opticalsurface area closer to the outside, the optical surface area closer tothe outside is the area on the peripheral side stated above.(19) In the optical pickup device described in (19), a diffractivesection where ring-shaped diffractive zones are formed is formed on theoptical surface area closer to the optical axis, and average pitch P inof the ring-shaped diffractive zones satisfies the following expression,when n^(th) order light represents a diffracted light with a maximumamount of light generated by the diffractive structure from a light fluxpassing through the diffractive structure on the optical surface areacloser to the light source among light fluxes emitted from the lightsource, and f represents a focal length of the objective lens.

3.00×10⁻³ ≦Pin/(|n|·f)≦8.6×10⁻²  (23)

(20) In the optical pickup device described in (20), the optical surfacearea closer to the outside has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium.(21) In the optical pickup device described in (21), the optical surfacearea closer to the optical axis has a function to correct sphericalaberration for thickness t (t₁<t<t₂) of a transparent base board.(22) In the optical pickup device described in (22), when recording orreproducing information for the first optical information recordingmedium, the optical surface area closer to the optical axis makes alight flux passing through the optical surface area closer to theoptical axis to have under spherical aberration, and when recording orreproducing information for the second optical information recordingmedium, the optical surface area closer to the optical axis makes alight flux passing through the optical surface area closer to theoptical axis to have over spherical aberration.(23) In the optical pickup device described in (23), when recording orreproducing information for the second optical information recordingmedium, the following expression is satisfied under the assumption thatthe optical surface area closer to the optical axis is formed within arange of the shortest distance NAH mm from the optical axis, when anecessary numerical aperture is represented by NA₂ and a focal length ofthe objective lens is represented by f₂.

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (24).

(24) In the optical pickup device described in (24), the followingexpression is satisfied by image forming magnification m1 of theobjective lens in conducting recording or reproduction of informationfor the first optical information recording medium.

−½≦m1≦− 1/7.5  (25)

(25) In the optical pickup device described in (25), image formingmagnification m2 of the objective lens in conducting recording orreproduction of information for the second optical information recordingmedium is nearly the same as m1.(26) The optical pickup device described in (26) is represented by anoptical pickup device having therein a first light source and a secondlight source each being different in terms of wavelength and alight-converging optical system including an objective lens forconverging a light fluxes emitted from the first and the second lightsources on an information recording surface of an optical informationrecording medium, and being capable of conducting recording and/orreproducing of information for a first optical information recordingmedium in which a thickness of a transparent base board is t₁ by usingthe first light source and the light-converging optical system and ofconducting recording and/or reproducing of information for a secondoptical information recording medium in which a thickness of atransparent base board is t₂ (t₁<t₂) by using the second light sourceand the light-converging optical system wherein the objective lens is aplastic lens, and when recording or reproducing information for thefirst optical information recording medium, a divergent light fluxemitted from the first light source enters the objective lens, and thefollowing conditional expression is satisfied when λ1 represents awavelength of the first light source, δSA₃/δU represents a change ofspherical aberration for object image distance change δU (|δU|≦0.5 mm)and δSA₄/δT represents a change of spherical aberration for temperaturechange δT (|δT|≦30° C.).

|δSA ₃ /δU|·|δU|+|δSA ₄ /δT|·|δT|≦0.07λ1rms  (26)

and when recording or reproducing information for the second opticalinformation recording medium, a divergent light flux emitted from thesecond light source enters the objective lens, and the followingconditional expression is satisfied when λ2 represents a wavelength ofthe second light source, δSA₅/δU represents a change of sphericalaberration for object image distance change δU (|δU|≦0.5 mm) and δSA₆/δTrepresents a change of spherical aberration for temperature change δT(|δT|≦30° C.).

|δSA ₅ /δU|·|δU|+|δSA ₆ /δT|·|δT|≦0.07λ2rms  (27)

In the optical pickup device described in (26), when the sum total of|δSA₃/δU|·|δU| and |δSA₄/δT|·|δT| and the sum total of |δSA₅/δU|·|δU|and |δSA₆/δT|·|δT| are looked and each sum total is made to be not morethan 0.07 λ1rms and 0.07 λ2rms respectively by providing the diffractivestructure on the objective lens, for example, it is possible to conductproperly recording or reproducing of information for two opticalinformation recording media even under the condition that divergentlight fluxes emitted from light sources being different in terms ofwavelength enter the objective lens, and it is possible to omit acollimator lens for forming a collimated light flux that enters theobjective lens, to attain cost reduction, and to make the structure ofthe optical pickup device to be compact.

(27) The optical pickup device described in (27) wherein at least onesurface of the objective lens is provided with a diffractive structureon at least a peripheral area in an effective diameter, and thefollowing conditional expression is satisfied when δSA1/δT represents achange of spherical aberration for temperature change ΔT in a light fluxwhich has passed the diffractive structure on the peripheral area amonglight fluxes emitted from the first light source.

|δSA1/δT|≦0.002λ1rms/° C.  (28)

(28) The optical pickup device described in (28) wherein δSA1/δTrepresenting a change of spherical aberration for temperature change ΔTin a light flux which has passed the diffractive structure on theperipheral area among light fluxes emitted from the first light sourcesatisfies the following conditional expression.

|δSA1/δT|≦0.0005λ1rms/° C.  (29)

(29) The optical pickup device described in (29) wherein the diffractivestructure on the peripheral area of the objective lens is a ring-shapeddiffractive zone, and an average pitch out of the ring-shapeddiffractive zone satisfies the following expression when n^(th) orderlight represents a diffracted light with a maximum amount of lightgenerated by the diffractive structure from a light flux passing throughthe diffractive structure on the peripheral area of the objective lensamong light fluxes emitted from the first light source, and f representsa focal length of the objective lens.

2.00×10⁻⁴ ≦Pout/(|n|·f)≦3.00×10⁻²  (30)

(30) The optical pickup device described in (30) wherein the averagepitch P out of the ring-shaped diffractive zone satisfies the followingexpression.

1.00×10⁻³ ≦Pout/(|n|·f)≦3.00×10⁻³  (31)

(31) The optical pickup device described in (31) wherein the averagepitch P out of the ring-shaped diffractive zone satisfies the followingexpression.

3.00×10⁻³ ≦Pout/(|n|·f)≦8.00×10⁻³  (32)

(32) The optical pickup device described in (32) wherein the opticalsurface of the objective lens is composed of three or more kinds ofoptical surface areas arranged in the direction perpendicular to anoptical axis, and when the three or more kinds of optical surface areasare represented by an optical surface area closer to the optical axis,an intermediate optical surface area and an optical surface area closerto the outside, all arranged in this order from the optical axis side,the optical surface area closer to the outside is the area on theperipheral side stated above.(33) The optical pickup device described in (32) wherein sphericalaberration is discontinuous in at least one of a boundary between theoptical surface area closer to the optical axis and the intermediateoptical surface and a boundary between the intermediate optical surfacearea and the optical surface area closer to the outside.(34) The optical pickup device described in (34), wherein a diffractivesection where ring-shaped diffractive zones are formed is formed on theoptical surface area closer to the optical axis, and average pitch P inof the ring-shaped diffractive zones satisfies the following expression,when n^(th) order light represents a diffracted light with a maximumamount of light generated by the diffractive structure from a light fluxpassing through the diffractive structure on the optical surface areacloser to the second light source among light fluxes emitted from thesecond light source, and f represents a focal length of the objectivelens.

3.00×10⁻³ ≦Pin/(|n|·f)≦8.0×10⁻²  (33)

(35) The optical pickup device described in (35), the optical surfacearea closer to the outside has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium stated above.(36) The optical pickup device described in (36), wherein when recordingor reproducing information for the first optical information recordingmedium, spherical aberration of the light flux passing through theintermediate optical surface area is made to be discontinuous and to beflare component, for spherical aberration of the light flux passingthrough the optical surface area closer to the outside, while whenrecording or reproducing information for the second optical informationrecording medium, the light flux passing through the intermediateoptical surface area is used.(37) The optical pickup device described in (37), wherein theintermediate optical surface area has a function to correct sphericalaberration for thickness t (t₁<t<t₂) of a transparent base board.(38) The optical pickup device described in (38), wherein when recordingor reproducing information for the first optical information recordingmedium, a light flux passing mainly through the optical surface areacloser to the optical axis and the optical surface area closer to theoutside is used, while when recording or reproducing information for thesecond optical information recording medium, a light flux passing mainlythrough the optical surface area closer to the optical axis and theintermediate optical surface area is used.(39) In the optical pickup device described in (39), when recording orreproducing information for the second optical information recordingmedium, the following expressions are satisfied under the assumptionthat the intermediate optical surface area is formed within a range fromthe shortest distance from an optical axis NAH mm to NAL mm when anecessary numerical aperture is represented by NA₂ and a focal length ofthe objective lens is represented by f₂.

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (34)

(NA ₂−0.20)f ₂ ≦NAL≦(NA ₂−0.04)f ₂  (35)

(40) The optical pickup device described in (40), wherein when recordingor reproducing information for the first optical information recordingmedium, a light flux passing through the intermediate optical surfacearea is made to have over spherical aberration.(41) The optical pickup device described in (41), wherein the opticalsurface area closer to the optical axis has a function to correctspherical aberration when recording or reproducing information for thefirst optical information recording medium.(42) The optical pickup device described in (42), wherein the opticalsurface area closer to the optical axis has a function to correcttemperature characteristics when recording or reproducing informationfor the first optical information recording medium.(43) The optical pickup device described in (43), wherein the opticalsurface of the objective lens is composed of two or more kinds ofoptical surface areas arranged in the direction perpendicular to anoptical axis, and when the two kinds of optical surface areas arerepresented by an optical surface area closer to the optical axis and anoptical surface area closer to the outside, the optical surface areacloser to the outside is the area on the peripheral side stated above.(44) The optical pickup device described in (44), wherein a diffractivesection where ring-shaped diffractive zones are formed is formed on theoptical surface area closer to the optical axis, average pitch P in ofthe ring-shaped diffractive zones satisfies the following expression,when n^(th) order light represents a diffracted light with a maximumamount of light generated by the diffractive structure from a light fluxpassing through the diffractive structure on the optical surface areacloser to the second light source among light fluxes emitted from thesecond light source, and f represents a focal length of the objectivelens.

3.00×10⁻³ ≦Pin/(|n|·f)≦8.0×10⁻²  (35)

(45) The optical pickup device described in (45), wherein the opticalsurface area closer to the outside has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium.(46) The optical pickup device described in (46), wherein the opticalsurface area closer to the optical axis has a function to correctspherical aberration for thickness t of a transparent base board.(47) The optical pickup device described in (47), wherein the opticalsurface area closer to the optical axis has a function to correctspherical aberration for a light flux passing through that opticalsurface area when recording or reproducing information for the secondoptical information recording medium, while the optical surface areacloser to the outside has a function to make the light flux passingthrough that optical surface area to be a flare component when recordingor reproducing information for the second optical information recordingmedium.(48) The optical pickup device described in (48), when recording orreproducing information for the second optical information recordingmedium, the following expression is satisfied under the assumption thatthe optical surface area closer to the optical axis is formed within arange of the shortest distance NAH mm from the optical axis, when anecessary numerical aperture is represented by NA₂ and a focal length ofthe objective lens is represented by f₂

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (36)

(49) The optical pickup device described in (49), wherein the followingexpression is satisfied by image forming magnification m1 of theobjective lens in conducting recording or reproduction of informationfor the first optical information recording medium.

−½≦m1≦− 1/7.5  (37)

(50) The optical pickup device described in 50), wherein image formingmagnification m2 of the objective lens in conducting recording orreproduction of information for the second optical information recordingmedium is nearly the same as m1.(51) The optical pickup device described in (51), wherein there areprovided a light source and a light-converging optical system includingthe objective lens for converging a divergent light flux that is emittedfrom the light source and enters objective lens on an informationrecording surface of an optical information recording medium, and theobjective lens of the optical pickup device capable of recording and/orreproducing information for the first optical information recordingmedium having a t1-thick transparent base board and for the secondoptical information recording medium having a t2-thick transparent baseboard (t₁<t₂) is a plastic lens and at least one side thereof isprovided with a diffractive structure on at least a peripheral areawithin an effective diameter, and thus, the following expression issatisfied, when δSA1/δT represents a change in spherical aberration fortemperature change δT in a light flux passing through the diffractivestructure on the peripheral area among light fluxes emitted from thelight source, and λ represents a wavelength of the light source.

|δSA1/δT|≦0.002λrms/° C.  (38)

In the objective lens described in (51), by providing the diffractivestructure that satisfies the expression (38) on the aforesaid peripheralarea, it is possible to conduct properly recording or reproducing ofinformation for two optical information recording media, even under thecondition that the objective lens is arranged on the optical pickupdevice and a divergent light flux emitted from the light source entersthe objective lens, thus, it is possible to omit a collimator lens forforming a collimated light flux that enters the objective lens, toattain cost reduction, and to make the structure of the optical pickupdevice to be compact.

(52) The objective lens described in (52), wherein δSA1/δT representinga change of spherical aberration for temperature change δT in a lightflux which has passed the diffractive structure on the peripheral areaamong light fluxes emitted from the light source satisfies the followingconditional expression.

|δSA1/δT|≦0.0005λrms/° C.  (39)

(53) The objective lens described in (53) wherein the diffractivestructure on the peripheral area of the objective lens is a ring-shapeddiffractive zone, and average pitch P out of the ring-shaped diffractivezone satisfies the following expression, when n^(th) order lightrepresents a diffracted light with the greatest amount of lightgenerated by the diffractive structure and by a light flux passingthrough the diffractive structure on the peripheral area of the objectlens among light fluxes emitted from the light source, and f representsa focal length of the objective lens.

2.00×10⁻⁴ ≦Pout/(|n|·f)≦3.00×10⁻²  (40)

(54) The objective lens described in (54) wherein average pitch P out ofthe ring-shaped diffractive zone satisfies the following expression.

1.00×10⁻³ ≦Pout/(|n|·f)≦3.00×10⁻³  (41)

(55) The objective lens described in (54) wherein average pitch P out ofthe ring-shaped diffractive zone satisfies the following expression.

3.00×10⁻³ ≦Pout/(|n|·f)≦8.00×10⁻³  (42)

(56) The objective lens described in (56) wherein the optical surface ofthe objective lens is composed of three or more types of optical surfaceareas arranged in the direction perpendicular to the optical axis, andwhen the three types of optical surface areas are represented by anoptical surface area closer to the optical axis, an intermediate opticalsurface area and an optical surface area closer to the outside, in thisorder from the optical axis side, the optical surface area closer to theoutside is the aforesaid peripheral area.(57) The objective lens described in (57) wherein spherical aberrationis discontinuous in at least one of a boundary between the opticalsurface area closer to the optical axis and the intermediate opticalsurface and a boundary between the intermediate optical surface area andthe optical surface area closer to the outside.(58) The objective lens described in (58) wherein a diffractive sectionhaving thereon a ring-shaped diffractive zone is formed on the opticalsurface area closer to the optical axis, and average pitch P in of thering-shaped diffractive zone satisfies the following expression, whenn^(th) order light represents a diffracted light with the greatestamount of light generated by the diffractive structure and by a lightflux passing through the diffractive structure on the optical surfacearea closer to the light source among light fluxes emitted from thelight source, and f represents a focal length of the objective lens.

3.00×10⁻³ ≦Pin/(|n|·f)≦8.00×10⁻²  (43)

(59) The objective lens described in (59) wherein the optical surfacearea closer to the outside has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium stated above.(60) The objective lens described in (60), wherein when recording orreproducing information for the first optical information recordingmedium, spherical aberration of the light flux passing through theintermediate optical surface area is made to be discontinuous and to beflare component, for spherical aberration of the light flux passingthrough the optical surface area closer to the outside, while whenrecording or reproducing information for the second optical informationrecording medium, the light flux passing through the intermediateoptical surface area is used.(61) The objective lens described in (61), wherein the intermediateoptical surface area has a function to correct spherical aberration forthickness t (t₁<t<t₂) of a transparent base board.(62) The objective lens described in (62), wherein when recording orreproducing information for the first optical information recordingmedium, a light flux passing mainly through the optical surface areacloser to the optical axis and the optical surface area closer to theoutside is used, while when recording or reproducing information for thesecond optical information recording medium, a light flux passing mainlythrough the optical surface area closer to the optical axis and theintermediate optical surface area is used.(63) The objective lens described in (63), wherein when recording orreproducing information for the second optical information recordingmedium, the following expressions are satisfied under the assumptionthat the intermediate optical surface area is formed within a range fromthe shortest distance from an optical axis NAH mm to NAL mm when anecessary numerical aperture is represented by NA₂ and a focal length ofthe objective lens is represented by f₂.

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (44)

(NA ₂−0.20)f ₂ ≦NAL≦(NA ₂−0.04)f ₂  (45)

(64) The objective lens described in (64), wherein when recording orreproducing information for the first optical information recordingmedium, a light flux passing through the intermediate optical surfacearea is made to have under spherical aberration.(65) The objective lens described in (65), wherein the optical surfacearea closer to the optical axis has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium.(66) The objective lens described in (66), wherein the optical surfacearea closer to the optical axis has a function to correct temperaturecharacteristics when recording or reproducing information for the firstoptical information recording medium.(67) The objective lens described in (67), wherein the optical surfaceof the objective lens is composed of two or more kinds of opticalsurface areas arranged in the direction perpendicular to an opticalaxis, and when the two kinds of optical surface areas are represented byan optical surface area closer to the optical axis and an opticalsurface area closer to the outside, the optical surface area closer tothe outside is the area on the peripheral side stated above.(68) The objective lens described in (68), wherein a diffractive sectionwhere ring-shaped diffractive zones are formed is formed on the opticalsurface area closer to the optical axis, average pitch P in of thering-shaped diffractive zones satisfies the following expression, whenn^(th) order light represents a diffracted light with a maximum amountof light generated by the diffractive structure from a light fluxpassing through the diffractive structure on the optical surface areacloser to the light source among light fluxes emitted from the lightsource, and f represents a focal length of the objective lens.

3.00×10⁻³ ≦Pin/(|n|·f)≦8.0×10⁻²  (46)

(69) The objective lens described in (69), wherein the optical surfacearea closer to the outside has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium.(70) The objective lens described in (70), wherein the optical surfacearea closer to the optical axis has a function to correct sphericalaberration for thickness t (t₁<t<t₂) of a transparent base board.(71) The objective lens described in (71), wherein when recording orreproducing information for the first optical information recordingmedium, the optical surface area closer to the optical axis makes alight flux passing through the optical surface area closer to theoptical axis to have under spherical aberration, and when recording orreproducing information for the second optical information recordingmedium, the optical surface area closer to the optical axis makes alight flux passing through the optical surface area closer to theoptical axis to have over spherical aberration.(72) The objective lens described in (72), wherein when recording orreproducing information for the second optical information recordingmedium, the following expression is satisfied under the assumption thatthe optical surface area closer to the optical axis is formed within arange of the shortest distance NAH mm from the optical axis, when anecessary numerical aperture is represented by NA₂ and a focal length ofthe objective lens is represented by f₂.

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (47)

(73) The objective lens described in (73), wherein the followingexpression is satisfied by image forming magnification m1 of theobjective lens in conducting recording or reproduction of informationfor the first optical information recording medium.

−½≦m1≦− 1/7.5  (48)

(74) The objective lens described in (74), wherein image formingmagnification m2 of the objective lens in conducting recording orreproduction of information for the second optical information recordingmedium is nearly the same as m1.(75) The objective lens described in (75) is represented by an objectivelens of an optical pickup device employing a first light source and asecond light source each being different in terms of wavelength and alight-converging optical system including the objective lens forconverging divergent light fluxes emitted from the first and the secondlight sources and enter the objective lens on an information recordingsurface of an optical information recording medium, and being capable ofconducting recording and/or reproducing of information for a firstoptical information recording medium in which a thickness of atransparent base board is t₁, and of conducting recording and/orreproducing of information for a second optical information recordingmedium in which a thickness of a transparent base board is t₂ (t₁<t₂),wherein the objective lens is a plastic lens, and at least one side ofthe objective lens is provided with a diffractive structure on at leasta peripheral area within an effective diameter, and the followingexpression is satisfied, when δSA1/δT represents a change in sphericalaberration for temperature change δT in a light flux passing through thediffractive structure on the peripheral area among light fluxes emittedfrom the first light source, and λ1 represents a wavelength of the firstlight source.

|δSA1/δT|≦0.002λ1rms/° C.  (49)

In the objective lens described in (75), by providing the diffractivestructure that satisfies the expression (49) on the aforesaid peripheralarea, it is possible to conduct properly recording or reproducing ofinformation for two optical information recording media, even under thecondition that the objective lens is arranged on the optical pickupdevice and a divergent light flux emitted from the light source having adifferent wavelength enters the objective lens, thus, it is possible toomit a collimator lens for forming a collimated light flux that entersthe objective lens, to attain cost reduction, and to make the structureof the optical pickup device to be compact.

(76) The objective lens described in (76), wherein δSA1/δT representinga change of spherical aberration for temperature change δT in a lightflux which has passed the diffractive structure on the peripheral areaamong light fluxes emitted from the first light source satisfies thefollowing conditional expression.

|δSA1/δT|≦0.0005λ1rms/° C.  (50)

(77) The objective lens described in (77) wherein the diffractivestructure on the peripheral area of the objective lens is a ring-shapeddiffractive zone, and average pitch P out of the ring-shaped diffractivezone satisfies the following expression, when n-th order lightrepresents a diffracted light with the greatest amount of lightgenerated by the diffractive structure and by a light flux passingthrough the diffractive structure on the peripheral area of the objectlens among light fluxes emitted from the first light source, and frepresents a focal length of the objective lens.

2.00×10⁻⁴ ≦Pout/(|n|·f)≦3.00×10⁻²  (51)

(78) The objective lens described in (78) wherein average pitch P out ofthe ring-shaped diffractive zone satisfies the following expression.

1.00×10⁻³ ≦Pout/(|n|·f)≦3.00×10⁻³  (52)

(79) The objective lens described in (79) wherein average pitch P out ofthe ring-shaped diffractive zone satisfies the following expression.

3.00×10⁻³ ≦Pout/(|n|·f)≦8.00×10⁻³  (53)

(80) The objective lens described in (80) wherein the optical surface ofthe objective lens is composed of three or more types of optical surfaceareas arranged in the direction perpendicular to the optical axis, andwhen the three types of optical surface areas are represented by anoptical surface area closer to the optical axis, an intermediate opticalsurface area and an optical surface area closer to the outside, in thisorder from the optical axis side, the optical surface area closer to theoutside is the aforesaid peripheral area.(81) The objective lens described in (81) wherein spherical aberrationis discontinuous in at least one of a boundary between the opticalsurface area closer to the optical axis and the intermediate opticalsurface and a boundary between the intermediate optical surface area andthe optical surface area closer to the outside.(82) The objective lens described in (82) wherein a diffractive sectionhaving thereon a ring-shaped diffractive zone is formed on the opticalsurface area closer to the optical axis, and average pitch P in of thering-shaped diffractive zone satisfies the following expression, whenn^(th) order light represents a diffracted light with the greatestamount of light generated by the diffractive structure and by a lightflux passing through the diffractive structure on the optical surfacearea closer to the second light source among light fluxes emitted fromthe second light source, and f represents a focal length of theobjective lens.

3.00×10⁻³ ≦Pin/(|n|·f)≦8.00×10⁻²  (54)

(83) The objective lens described in (83) wherein the optical surfacearea closer to the outside has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium stated above.(84) The objective lens described in (84), wherein when recording orreproducing information for the first optical information recordingmedium, spherical aberration of the light flux passing through theintermediate optical surface area is made to be discontinuous and to beflare component, for spherical aberration of the light flux passingthrough the optical surface area closer to the outside, while whenrecording or reproducing information for the second optical informationrecording medium, the light flux passing through the intermediateoptical surface area is used.(85) The objective lens described in (85), wherein the intermediateoptical surface area has a function to correct spherical aberration forthickness t (t₁<t<t₂) of a transparent base board.(86) The objective lens described in (86), wherein when recording orreproducing information for the first optical information recordingmedium, a light flux passing mainly through the optical surface areacloser to the optical axis and the optical surface area closer to theoutside is used, while when recording or reproducing information for thesecond optical information recording medium, a light flux passing mainlythrough the optical surface area closer to the optical axis and theintermediate optical surface area is used.(87) The objective lens described in (87), wherein when recording orreproducing information for the second optical information recordingmedium, the following expressions are satisfied under the assumptionthat the intermediate optical surface area is formed within a range fromthe shortest distance from an optical axis NAH mm to NAL mm when anecessary numerical aperture is represented by NA₂ and a focal length ofthe objective lens is represented by f₂.

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (55)

(NA ₂−0.20)f ₂ ≦NAL≦(NA ₂−0.04)f ₂  (56)

(88) The objective lens described in (88), wherein when recording orreproducing information for the first optical information recordingmedium, a light flux passing through the intermediate optical surfacearea is made to have over spherical aberration.(89) The objective lens described in (89), wherein the optical surfacearea closer to the optical axis has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium.(90) The objective lens described in (90), wherein the optical surfacearea closer to the optical axis has a function to correct temperaturecharacteristics when recording or reproducing information for the firstoptical information recording medium.(91) The objective lens described in (91), wherein the optical surfaceof the objective lens is composed of two or more kinds of opticalsurface areas arranged in the direction perpendicular to an opticalaxis, and when the two kinds of optical surface areas are represented byan optical surface area closer to the optical axis and an opticalsurface area closer to the outside, the optical surface area closer tothe outside is the area on the peripheral side stated above.(92) The objective lens described in (92), wherein a diffractive sectionwhere ring-shaped diffractive zones are formed is formed on the opticalsurface area closer to the optical axis, average pitch P in of thering-shaped diffractive zones satisfies the following expression, whenn-th order light represents a diffracted light with a maximum amount oflight generated by the diffractive structure from a light flux passingthrough the diffractive structure on the optical surface area closer tothe second light source among light fluxes emitted from the second lightsource, and f represents a focal length of the objective lens.

3.00×10³ ≦Pin/(|n|·f)≦8.0×10⁻²  (57)

(93) The objective lens described in (93), wherein the optical surfacearea closer to the outside has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium.(94) The objective lens described in (94), wherein the optical surfacearea closer to the optical axis has a function to correct sphericalaberration for thickness t of a transparent base board.(95) The objective lens described in (95), wherein when recording orreproducing information for the second optical information recordingmedium, the optical surface area closer to the optical axis has afunction to correct spherical aberration for the light flux passingthrough that optical surface area, while when recording or reproducinginformation for the second optical information recording medium, theoptical surface area closer to the outside has a function to make thelight flux passing through that optical surface to be flare components.(96) The objective lens described in (96), wherein when recording orreproducing information for the second optical information recordingmedium, the following expression is satisfied under the assumption thatthe optical surface area closer to the optical axis is formed within arange of the shortest distance NAH mm from the optical axis, when anecessary numerical aperture is represented by NA₂ and a focal length ofthe objective lens is represented by f₂.

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (58)

(97) The objective lens described in (97), wherein the followingexpression is satisfied by image forming magnification m1 of theobjective lens in conducting recording or reproduction of informationfor the first optical information recording medium.

−½≦m1≦− 1/7.5  (59)

(98) The objective lens described in (98), wherein image formingmagnification m2 of the objective lens in conducting recording orreproduction of information for the second optical information recordingmedium is nearly the same as m1.(99) The objective lens described in (99) is represented by an objectivelens of an optical pickup device having a light source and alight-converging optical system including the objective lens forconverging a divergent light flux that is emitted from the light sourceand enters the objective lens on an information recording surface of anoptical information recording medium, and being an objective lens of anoptical pickup device capable of conducting recording and/or reproducingof information for a first optical information recording medium in whicha thickness of a transparent base board is t₁; and for a second opticalinformation recording medium in which a thickness of a transparent baseboard is t₂ (t₁<t₂), wherein the objective lens is a plastic lens, andat least one side of the objective lens is provided with at least twotypes of areas within an effective diameter in the direction from theoptical axis of the objective lens toward the periphery, and thediffractive structure is provided on at least an area on the peripheralportion within the effective diameter, and the following expression issatisfied, when δSA1/δT represents a change in spherical aberration fortemperature change δT in a light flux passing through the diffractivestructure on the peripheral area among light fluxes emitted from thelight source, and λ represents a wavelength of the light source, and anarea inside the peripheral area is designed to correct sphericalaberration for recording or reproducing information for the secondoptical information recording medium.

|δSA1/δT|≦0.002λrms/° C.

In the objective lens described in (99), change δSA1/δT of sphericalaberration for a temperature change is corrected by the diffractivestructure on the aforesaid peripheral area in recording or reproducingof information for the first optical information recording medium, andspherical aberration is corrected by the area inside the peripheral areain recording or reproducing of information for the second opticalinformation recording medium, and therefore, it is possible to conductproperly recording or reproducing of information for both opticalinformation recording media, even under the condition that the objectivelens is arranged on the optical pickup device and divergent light fluxesemitted from the light sources enter the objective lens, thus, it ispossible to omit a collimator lens for forming a collimated light fluxthat enters the objective lens, to attain cost reduction, and to makethe structure of the optical pickup device to be compact.

(100) The objective lens described in (100), wherein δSA1/δTrepresenting a change of spherical aberration for temperature change δTin a light flux which has passed the diffractive structure on theperipheral area among light fluxes emitted from the light sourcesatisfies the following conditional expression.

δSA1/δT|≦0.0005λrms/° C.  (60)

(101) The objective lens described in (101) wherein the diffractivestructure on the peripheral area of the objective lens is a ring-shapeddiffractive zone, and average pitch P out of the ring-shaped diffractivezone satisfies the following expression, when n-th order lightrepresents a diffracted light with the greatest amount of lightgenerated by the diffractive structure and by a light flux passingthrough the diffractive structure on the peripheral area of the objectlens among light fluxes emitted from the light source, and f representsa focal length of the objective lens.

2.00×10⁻⁴ ≦Pout/(|n|·f)≦3.00×10⁻²  (61)

(102) The objective lens described in (102) wherein average pitch P outof the ring-shaped diffractive zone satisfies the following expression.

1.00×10⁻³ ≦Pout/(|n|·f)≦3.00×10⁻³  (62)

(103) The objective lens described in (103) wherein average pitch P outof the ring-shaped diffractive zone satisfies the following expression.

3.00×10⁻³ ≦Pout/(|n|·f)≦8.00×10⁻³  (63)

(104) The objective lens described in (104) wherein the optical surfaceof the objective lens is composed of three or more kinds of opticalsurface areas arranged in the direction perpendicular to an opticalaxis, and when the three or more kinds of optical surface areas arerepresented by an optical surface area closer to the optical axis, anintermediate optical surface area and an optical surface area closer tothe outside, all arranged in this order from the optical axis side, theoptical surface area closer to the outside is the area on the peripheralside stated above.(105) The objective lens described in (105) wherein spherical aberrationis discontinuous in at least one of a boundary between the opticalsurface area closer to the optical axis, and the intermediate opticalsurface and a boundary between the intermediate optical surface area andthe optical surface area closer to the outside.(106) The objective lens described in (106) wherein a diffractivesection having thereon a ring-shaped diffractive zone is formed on theoptical surface area closer to the optical axis, and average pitch P inof the ring-shaped diffractive zone satisfies the following expression,when n^(th) order light represents a diffracted light with the greatestamount of light generated by the diffractive structure and by a lightflux passing through the diffractive structure on the optical surfacearea closer to the light source among light fluxes emitted from thelight source, and f represents a focal length of the objective lens.

3.00×10⁻³ ≦Pin/(|n|·f)≦8.00×10⁻²  (64)

(107) The objective lens described in (107) wherein the optical surfacearea closer to the outside has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium stated above.(108) The objective lens described in (108), wherein when recording orreproducing information for the first optical information recordingmedium, spherical aberration of the light flux passing through theintermediate optical surface area is made to be discontinuous and to beflare component, for spherical aberration of the light flux passingthrough the optical surface area closer to the outside, while whenrecording or reproducing information for the second optical informationrecording medium, the light flux passing through the intermediateoptical surface area is used.(109) The objective lens described in (109), wherein the intermediateoptical surface area has a function to correct spherical aberration forthickness t (t₁<t<t₂) of a transparent base board.(110) The objective lens described in (110), wherein when recording orreproducing information for the first optical information recordingmedium, a light flux passing mainly through the optical surface areacloser to the optical axis and the optical surface area closer to theoutside is used, while when recording or reproducing information for thesecond optical information recording medium, a light flux passing mainlythrough the optical surface area closer to the optical axis and theintermediate optical surface area is used.(111) The objective lens described in (111), wherein when recording orreproducing information for the second optical information recordingmedium, the following expressions are satisfied under the assumptionthat the intermediate optical surface area is formed within a range fromthe shortest distance from an optical axis NAH mm to NAL mm when anecessary numerical aperture is represented by NA₂ and a focal length ofthe objective lens is represented by f₂.

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (65)

(NA ₂−0.20)f ₂ ≦NAL≦(NA ₂−0.04)f ₂  (66)

(112) The objective lens described in (112), wherein when recording orreproducing information for the first optical information recordingmedium, a light flux passing through the intermediate optical surfacearea is made to have under spherical aberration.(113) The objective lens described in (113), wherein the optical surfacearea closer to the optical axis has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium.(114) The objective lens described in (114), wherein the optical surfacearea closer to the optical axis has a function to correct temperaturecharacteristics when recording or reproducing information for the firstoptical information recording medium.(115) The objective lens described in (115), wherein the optical surfaceof the objective lens is composed of two or more kinds of opticalsurface areas arranged in the direction perpendicular to an opticalaxis, and when the two kinds of optical surface areas are represented byan optical surface area closer to the optical axis and an opticalsurface area closer to the outside, the optical surface area closer tothe outside is the area on the peripheral side stated above.(116) The objective lens described in (116), wherein a diffractivesection where ring-shaped diffractive zones are formed is formed on theoptical surface area closer to the optical axis, and average pitch P inof the ring-shaped diffractive zones satisfies the following expression,when n^(th) order light represents a diffracted light with a maximumamount of light generated by the diffractive structure from a light fluxpassing through the diffractive structure on the optical surface areacloser to the light source among light fluxes emitted from the lightsource, and f represents a focal length of the objective lens.

3.00×10⁻³ ≦Pin/(|n|·f)≦8.0×10⁻²  (67)

(117) The objective lens described in (117), wherein the optical surfacearea closer to the outside has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium.(118) The objective lens described in (118), wherein the optical surfacearea closer to the optical axis has a function to correct sphericalaberration for thickness t (t₁<t<t₂) of a transparent base board.(119) The objective lens described in (119), wherein when recording orreproducing information for the first optical information recordingmedium, the optical surface area closer to the optical axis makes alight flux passing through the optical surface area closer to theoptical axis to have under spherical aberration, and when recording orreproducing information for the second optical information recordingmedium, the optical surface area closer to the optical axis makes alight flux passing through the optical surface area closer to theoptical axis to have over spherical aberration.(120) The objective lens described in (120), wherein when recording orreproducing information for the second optical information recordingmedium, the following expression is satisfied under the assumption thatthe optical surface area closer to the optical axis is formed within arange of the shortest distance NAH mm from the optical axis, when anecessary numerical aperture is represented by NA₂ and a focal length ofthe objective lens is represented by f₂.

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (68)

(121) The objective lens described in (121), wherein the followingexpression is satisfied by image forming magnification m1 of theobjective lens in conducting recording or reproduction of informationfor the first optical information recording medium.

−½≦m1≦− 1/7.5  (69)

(122) The objective lens described in (122), wherein image formingmagnification m2 of the objective lens in conducting recording orreproduction of information for the second optical information recordingmedium is nearly the same as m1.(123) The objective lens described in (123) is represented by anobjective lens of an optical pickup device having a first light sourceand a second light source each being different each other in terms ofwavelength and a light-converging optical system including the objectivelens for converging divergent light fluxes emitted from the first andthe second light sources and enter the objective lens on an informationrecording surface of an optical information recording medium, and beingcapable of conducting recording and/or reproducing of information for afirst optical information recording medium in which a thickness of atransparent base board is t₁ by using the first light source and thelight-converging optical system, and of conducting recording and/orreproducing of information for a second optical information recordingmedium in which a thickness of a transparent base a board is t₂ (t₁<t₂)by using the second light source and the light-converging opticalsystem, wherein the objective lens is a plastic lens, and at least oneside of the objective lens is provided with at least two types of areaswithin an effective diameter in the direction from the optical axis ofthe objective lens toward the periphery, and the diffractive structureis provided on at least an area on the peripheral portion within theeffective diameter, and the following expression is satisfied, whenδSA1/δT represents a change in spherical aberration for temperaturechange δT in a light flux passing through the diffractive structure onthe peripheral area among light fluxes emitted from the first lightsource, and λ represents a wavelength of the light source, and an areainside the peripheral area is designed to correct spherical aberrationfor recording or reproducing information for the second opticalinformation recording medium.

|δSA1/δT|≦0.002λrms/° C.  (70)

In the objective lens described in (123), change δSA1/δT of sphericalaberration for a temperature change is corrected by the diffractivestructure on the aforesaid peripheral area in recording or reproducingof information for the first optical information recording medium, andspherical aberration is corrected by the area inside the peripheral areain recording or reproducing of information for the second opticalinformation recording medium, and therefore, it is possible to conductproperly recording or reproducing of information for both opticalinformation recording media, even under the condition that the objectivelens is arranged on the optical pickup device and divergent light fluxeseach having a different light source wavelength respectively enter theobjective lens, thus, it is possible to omit a collimator lens forforming a collimated light flux that enters the objective lens, toattain cost reduction, and to make the structure of the optical pickupdevice to be compact.

(124) The objective lens described in (124), wherein δSA1/δTrepresenting a change of spherical aberration for temperature change δTin a light flux which has passed the diffractive structure on theperipheral area among light fluxes emitted from the first light sourcesatisfies the following conditional expression.

|δSA1/δT|≦0.0005λrms/° C.  (71)

(125) The objective lens described in (125) wherein the diffractivestructure on the peripheral area of the objective lens is a ring-shapeddiffractive zone, and average pitch P out of the ring-shaped diffractivezone satisfies the following expression, when n^(th) order lightrepresents a diffracted light with the greatest amount of lightgenerated by the diffractive structure and by a light flux passingthrough the diffractive structure on the peripheral area of the objectlens among light fluxes emitted from the first light source, and frepresents a focal length of the objective lens.

2.00×10⁻⁴ ≦Pout/(|n|·f)≦3.00×10⁻²  (72)

(126) The objective lens described in (126) wherein average pitch P outof the ring-shaped diffractive zone mentioned above satisfies thefollowing expression.

1.00×10⁻³ ≦Pout/(|n|·f)≦3.00×10⁻³  (73)

(127) The objective lens described in (127) wherein average pitch P outof the ring-shaped diffractive zone satisfies the following expression.

3.00×10⁻³ ≦Pout/(|n|·f)≦8.00×10⁻³  (74)

(128) The objective lens described in (128) wherein the optical surfaceof the objective lens is composed of three or more kinds of opticalsurface areas arranged in the direction perpendicular to an opticalaxis, and when the three or more kinds of optical surface areas arerepresented by an optical surface area closer to the optical axis, anintermediate optical surface area and an optical surface area closer tothe outside, all arranged in this order from the optical axis side, theoptical surface area closer to the outside is the area on the peripheralside stated above.(129) The objective lens described in (129) wherein spherical aberrationis discontinuous in at least one of a boundary between the opticalsurface area closer to the optical axis and the intermediate opticalsurface and a boundary between the intermediate optical surface area andthe optical surface area closer to the outside.(130) The objective lens described in (130) wherein a diffractivesection having thereon a ring-shaped diffractive zone is formed on theoptical surface area closer to the optical axis, and average pitch P inof the ring-shaped diffractive zone satisfies the following expression,when n^(th) order light represents a diffracted light with the greatestamount of light generated by the diffractive structure and by a lightflux passing through the diffractive structure on the optical surfacearea closer to the second light source among light fluxes emitted fromthe second light source, and f represents a focal length of theobjective lens.

3.00×10⁻³ ≦Pin/(|n|·f)≦8.00×10⁻²  (75)

(131) The objective lens described in (131) wherein the optical surfacearea closer to the outside has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium stated above.(132) The objective lens described in (132), wherein when recording orreproducing information for the first optical information recordingmedium, spherical aberration of the light flux passing through theintermediate optical surface area is made to be discontinuous and to beflare component, for spherical aberration of the light flux passingthrough the optical surface area closer to the outside, while whenrecording or reproducing information for the second optical informationrecording medium, the light flux passing through the intermediateoptical surface area is used.(133) The objective lens described in (133), wherein the intermediateoptical surface area has a function to correct spherical aberration forthickness t (t₁<t<t₂) of a transparent base board.(134) The objective lens described in (134), wherein when recording orreproducing information for the first optical information recordingmedium, a light flux passing mainly through the optical surface areacloser to the optical axis and the optical surface area closer to theoutside is used, while when recording or reproducing information for thesecond optical information recording medium, a light flux passing mainlythrough the optical surface area closer to the optical axis and theintermediate optical surface area is used.(135) The objective lens described in (135), wherein when recording orreproducing information for the second optical information recordingmedium, the following expressions are satisfied under the assumptionthat the intermediate optical surface area is formed within a range fromthe shortest distance from an optical axis NAH mm to NAL mm when anecessary numerical aperture is represented by NA₂ and a focal length ofthe objective lens is represented by f₂.

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (76)

(NA ₂−0.20)f ₂ ≦NAL≦(NA ₃−0.04)f ₂  (77)

(136) The objective lens described in (136), wherein when recording orreproducing information for the first optical information recordingmedium, a light flux passing through the intermediate optical surfacearea is made to have over spherical aberration.(137) The objective lens described in (137), wherein the optical surfacearea closer to the optical axis has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium.(138) The objective lens described in (138), wherein the optical surfacearea closer to the optical axis has a function to correct temperaturecharacteristics when recording or reproducing information for the firstoptical information recording medium.(139) The objective lens described in (139), wherein the optical surfaceof the objective lens is composed of two or more kinds of opticalsurface areas arranged in the direction perpendicular to an opticalaxis, and when the two kinds of optical surface areas are represented byan optical surface area closer to the optical axis and an opticalsurface area closer to the outside, the optical surface area closer tothe outside is the area on the peripheral side stated above.(140) The objective lens described in (140), wherein a diffractivesection where ring-shaped diffractive zones are formed is formed on theoptical surface area closer to the optical axis, and average pitch P inof the ring-shaped diffractive zones satisfies the following expression,when n^(th) order light represents a diffracted light with a maximumamount of light generated by the diffractive structure from a light fluxpassing through the diffractive structure on the optical surface areacloser to the second light source among light fluxes emitted from thesecond light source, and f represents a focal length of the objectivelens.

3.00×10⁻³ ≦Pin/(|n|·f)≦8.0×10⁻²  (78)

(141) The objective lens described in (141), wherein the optical surfacearea closer to the outside has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium.(142) The objective lens described in (142), wherein the optical surfacearea closer to the optical axis has a function to correct sphericalaberration for thickness t₁ of a transparent base board.(143) The objective lens described in (143), wherein when recording orreproducing information for the second optical information recordingmedium, the optical surface area closer to the optical axis has afunction to correct spherical aberration for the light flux passingthrough that optical surface area, while when recording or reproducinginformation for the second optical information recording medium, theoptical surface area closer to the outside has a function to make thelight flux passing through that optical surface area to be a flarecomponent.(144) The objective lens described in (144), wherein when recording orreproducing information for the second optical information recordingmedium, the following expression is satisfied under the assumption thatthe optical surface area closer to the optical axis is formed within arange of the shortest distance NAH mm from the optical axis, when anecessary numerical aperture is represented by NA₂ and a focal length ofthe objective lens is represented by f₂.

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (80)

(145) The objective lens described in (145), wherein the followingexpression is satisfied by image forming magnification m1 of theobjective lens in conducting recording or reproduction of informationfor the first optical information recording medium.

−½≦m1≦− 1/7.5  (81)

(146) The objective lens described in (146), wherein image formingmagnification m2 of the objective lens in conducting recording orreproduction of information for the second optical information recordingmedium is nearly the same as m1.(147) The objective lens described in (147) is represented by anobjective lens of an optical pickup device having a light source and alight-converging optical system including the objective lens forconverging a divergent light flux that is emitted from the light sourceand enters the objective lens on an information recording surface of anoptical information recording medium, and being an objective lens of anoptical pickup device capable of conducting recording and/or reproducingof information for a optical information recording medium in which athickness of a transparent base board is t₁, wherein the objective lensis a plastic lens, and at least one side of the objective lens isprovided with a diffractive structure on at least a peripheral areawithin an effective diameter, and the following expression is satisfied,when δSA1/δT represents a change in spherical aberration for temperaturechange δT in a light flux passing through the diffractive structure onthe peripheral area among light fluxes emitted from the light source,and λ represents a wavelength of the light source.

|δSA1/δT|≦0.002≦λrms/° C.  (82)

In the objective lens described in (147), change δSA1/δT of sphericalaberration for a temperature change is corrected properly by thediffractive structure on the aforesaid peripheral area in recording orreproducing of information for the first optical information recordingmedium, and therefore, it is possible to conduct properly recording orreproducing of information for both optical information recording media,even under the condition that the objective lens is arranged on theoptical pickup device and divergent light fluxes emitted from the lightsources enter the objective lens, thus, it is possible to omit acollimator lens for forming a collimated light flux that enters theobjective lens, to attain cost reduction, and to make the structure ofthe optical pickup device to be compact.

(148) The objective lens described in (148), wherein δSA1/δTrepresenting a change of spherical aberration for temperature change δTin a light flux which has passed the diffractive structure on theperipheral area among light fluxes emitted from the light sourcesatisfies the following conditional expression.

|δSA1/δT|≦0.0005λrms/° C.  (83)

(149) The objective lens described in (149) wherein the diffractivestructure on the peripheral area of the objective lens is a ring-shapeddiffractive zone, and average pitch P out of the ring-shaped diffractivezone satisfies the following expression, when n-th order lightrepresents a diffracted light with the greatest amount of lightgenerated by the diffractive structure and by a light flux passingthrough the diffractive structure on the peripheral area of the objectlens among light fluxes emitted from the light source, and f representsa focal length of the objective lens.

2.00×10⁻⁴ ≦Pout/(|n|·f)≦3.00×10⁻²  (84)

(150) The objective lens described in (150) wherein average pitch P outof the ring-shaped diffractive zone satisfies the following expression.

1.00×10⁻³ ≦Pout/(|n|·f)≦3.00×10⁻³  (85)

(151) The objective lens described in (151) wherein average pitch P outof the ring-shaped diffractive zone satisfies the following expression.

3.00×10⁻³ ≦Pout/(|n|·f)≦8.00×10⁻³  (86)

(152) The objective lens described in (152) wherein the optical surfaceof the objective lens is composed of three or more kinds of opticalsurface areas arranged in the direction perpendicular to an opticalaxis, and when the three or more kinds of optical surface areas arerepresented by an optical surface area closer to the optical axis, anintermediate optical surface area and an optical surface area closer tothe outside, all arranged in this order from the optical axis side, theoptical surface area closer to the outside is the area on the peripheralside stated above.(153) The objective lens described in (153) wherein spherical aberrationis discontinuous in at least one of a boundary between the opticalsurface area closer to the optical axis and the intermediate opticalsurface and a boundary between the intermediate optical surface area andthe optical surface area closer to the outside.(154) The objective lens described in (154) wherein a diffractivesection having thereon a ring-shaped diffractive zone is formed on theoptical surface area closer to the optical axis, and average pitch P inof the ring-shaped diffractive zone satisfies the following expression,when n^(th) order light represents a diffracted light with the greatestamount of light generated by the diffractive structure and by a lightflux passing through the diffractive structure on the optical surfacearea closer to the light source among light fluxes emitted from thelight source, and f represents a focal length of the objective lens.

3.00×10⁻³ ≦Pin/(|n|·f)≦8.00×10⁻²  (87)

(155) The objective lens described in (155) wherein the optical surfacearea closer to the outside has a function to correct sphericalaberration.(156) The objective lens described in (156), wherein when recording orreproducing information for the optical information recording medium,the following expressions are satisfied under the assumption that theintermediate optical surface area is formed within a range from theshortest distance from an optical axis NAH mm to NAL mm when a necessarynumerical aperture is represented by NA₂ and a focal length of theobjective lens is represented by f₂.

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (88)

(NA ₂−0.20)f ₂ ≦NAL≦(NA ₂−0.04)f ₂  (89)

(157) The objective lens described in (157), wherein the optical surfacearea closer to the optical axis has a function to correct sphericalaberration.(158) The objective lens described in (158), wherein the optical surfacearea closer to the optical axis has a function to correct temperaturecharacteristics.(159) The objective lens described in (159), wherein the optical surfaceof the objective lens is composed of two or more kinds of opticalsurface areas arranged in the direction perpendicular to an opticalaxis, and when the two kinds of optical surface areas are represented byan optical surface area closer to the optical axis and an opticalsurface area closer to the outside, the optical surface area closer tothe outside is the area on the peripheral side stated above.(160) The objective lens described in (160), wherein a diffractivesection where ring-shaped diffractive zones are formed is formed on theoptical surface area closer to the optical axis, and average pitch P inof the ring-shaped diffractive zones satisfies the following expression,when n-th order light represents a diffracted light with a maximumamount of light generated by the diffractive structure from a light fluxpassing through the diffractive structure on the optical surface areacloser to the light source among light fluxes emitted from the lightsource, and f represents a focal length of the objective lens.

3.00×10⁻³ ≦Pin/(|n|·f)≦8.0×10⁻²  (90)

(161) The objective lens described in (161), wherein the optical surfacearea closer to the outside has a function to correct sphericalaberration.(162) The objective lens described in (162), wherein when recording orreproducing information for the optical information recording medium,the following expression is satisfied under the assumption that theoptical surface area closer to the optical axis is formed within a rangeof the shortest distance from an optical axis NAH mm from the opticalaxis when a necessary numerical aperture is represented by NA₂ and afocal length of the objective lens is represented by f₂ and a focallength of the objective lens is represented by f₂.

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (91)

(163) The objective lens described in (163), wherein the followingexpression is satisfied by image forming magnification m1 of theobjective lens in conducting recording or reproduction of informationfor the first optical information recording medium.

−½≦m1≦− 1/7.5  (92)

(164) The objective lens described in (164), wherein image formingmagnification m2 of the objective lens in conducting recording orreproduction of information for the second optical information recordingmedium is nearly the same as m1.(165) The optical pickup device described in (165), wherein theobjective lens described in either one of (51) (164) is employed.(166) The objective lens described in (166) is represented by anobjective lens for conducting recording and/or reproducing ofinformation for the optical information recording medium by converginglight emitted from a light source on an information recording surface ofthe optical information recording medium through a transparent baseboard thereof, wherein a surface on at least one side of the objectivelens is constituted with at least two or more kinds of optical surfaceareas in the effective diameter of the objective lens, and a diffractivesection to utilize n^(th) order light on which a ring-shaped diffractivezone is formed is formed on an optical surface area that is outermost inthe direction perpendicular to the optical axis, or on the surface onthe other side through which a light flux passing through the outermostoptical surface area passes, and average pitch P out of the ring-shapeddiffractive zone satisfies the following expression when a focal lengthof the objective lens is represented by f.

2.00×10⁻⁴ ≦Pout/(|n|·f)≦3.50×10⁻³  (93)

In the foregoing, in the case of an objective lens where a divergentlight flux enters, for example, m shown in expression (1) is not zero,and an amount of change of spherical aberration for temperature changeis increased. Therefore, a ring-shaped diffractive zone is provided asin the objective lens described in (166), and its average pitch P out ismade to satisfy expression (93)., which makes it possible to control achange of spherical aberration for the temperature change and to obtainexcellent characteristics even when the divergent light flux enters.Thus, a collimator can be omitted, and compactness and low cost can beattained accordingly.

(167) The objective lens described in (167) wherein average pitch P outof the ring-shaped diffractive zone satisfies the following expression.

1.00×10⁻³ ≦Pout/(|n|·f)≦3.00×10⁻³  (94)

(168) The objective lens described in (168) wherein the optical surfaceon at least one side of the objective lens is constituted with three ormore types of optical surface areas arranged in the directionperpendicular to the optical axis, and an intermediate optical surfacearea among the aforesaid optical surface areas is provided with adiscontinuous section in terms of spherical aberration for at least oneoptical surface area among the outside and inside optical surface areas.(169) The objective lens described in (169) wherein at least one of therefraction section and the diffractive section is formed on theintermediate optical surface area.(170) The objective lens described in (170) wherein there is formed adiffractive section on which a ring-shaped diffractive zone is formed,on the optical surface area including an optical axis excluding theaforesaid intermediate optical surface area, and average pitch P in ofthat ring-shaped diffractive zone satisfies the following expression.

3.00×10⁻³ ≦Pin/(|n|·f)≦6.00×10⁻²  (95)

(171) The objective lens described in (171) wherein the surface on atleast one side of the objective lens is constituted with two types ofoptical surfaces and a diffractive section on which a ring-shapeddiffractive zone is formed is formed on the optical surface areaincluding an optical axis, and average pitch P in of that ring-shapeddiffractive zone satisfies the following expression.

3.00×10⁻³ ≦Pin/(|n|·f)≦6.00×10⁻²  (96)

(172) The objective lens described in (172) is characterized to be madeof plastic materials.(173) The objective lens described in (173) is represented by anobjective lens of an optical pickup device having a light sourceemitting light fluxes for the first optical information recording mediumhaving a t₁-thick transparent base board and for the second opticalinformation recording medium having a t₂-thick transparent base board(t₁<t₂) and a light-converging optical system including an objectivelens converging the light fluxes emitted from the light source on aninformation recording surface through the transparent base boards of thefirst and second optical information recording media, and conductingrecording and/or reproducing of information for each of the opticalinformation recording media, wherein a surface on at least one side ofthe objective lens is constituted with at least two or more kinds ofoptical surface areas in the effective diameter of the objective lens,and a diffractive section to utilize n^(th) order light on which aring-shaped diffractive zone is formed is formed on an optical surfacearea that is outermost in the direction perpendicular to the opticalaxis, or on the surface on the other side through which a light fluxpassing through the outermost optical surface area passes, and averagepitch P out of the ring-shaped diffractive zone satisfies the followingexpression when a focal length of the objective lens is represented byf.

2.00×10⁻⁴ ≦Pout/(|n|·f)≦3.50×10⁻³  (97)

In the case of an objective lens where a divergent light flux enters asstated above, m shown in expression (1) is not zero, and an amount ofchange of spherical aberration for temperature change is increasedaccordingly. Therefore, a ring-shaped diffractive zone is provided as inthe objective lens described in (173), and its average pitch P out ismade to satisfy expression (97), which makes it possible to control achange of spherical aberration for the temperature change and to obtainexcellent characteristics even when the divergent light flux enters.Incidentally, the optical pickup device employing the objective lensdescribed in (173) is capable of recording or reproducing informationfor optical information recording media in plural types, and therefore,it is possible to omit a collimator lens by using divergent lightfluxes, and to attain compactness and low cost of the apparatusaccordingly, which is preferable.

(174) The objective lens described in (174) wherein average pitch P outof the ring-shaped diffractive zone satisfies the following expression.

1.00×10⁻³ ≦Pout/(|n|·f)≦3.00×10⁻³  (96)

(175) The objective lens described in (175) wherein a divergent lightemitted from the light source enters the objective lens.(176) The objective lens described in (176), wherein the followingexpression is satisfied by image forming magnification m1 of theobjective lens in the course of conducting recording or reproduction ofinformation for the first optical information recording medium.

−½≦m1≦− 1/7.5  (99)

(177) The objective lens described in (177), wherein image formingmagnification m2 of the objective lens in conducting recording orreproduction of information for the second optical information recordingmedium is nearly the same as m1.(178) The objective lens described in (178), wherein the outermostoptical surface area has a function to correct spherical aberration whenrecording or reproducing information for the first optical informationrecording medium.(179) The objective lens described in (179) wherein the optical surfacearea on at least one side of the objective lens is composed of three ormore kinds of optical surface areas arranged in the directionperpendicular to an optical axis, and when recording or reproducinginformation for the first optical information recording medium,spherical aberration given to the light flux passing through theintermediate optical surface area is made to be discontinuous to be aflare component with respect to spherical aberration of the outermostoptical surface area, and when recording or reproducing information forthe second optical information recording medium, the light sourcepassing through the intermediate optical surface area is used.(180) The objective lens described in (180) wherein the intermediateoptical surface area has a function to correct spherical aberration forthickness t (t₁<t<t₂) of a transparent base board.(181) The objective lens described in (181) wherein light fluxes passingrespectively through the optical surface area mainly including anoptical axis and the outermost optical surface area are used whenrecording or reproducing information for the first optical informationrecording medium, and light fluxes passing respectively through theoptical surface area mainly including an optical axis and theintermediate optical surface area are used when recording or reproducinginformation for the second optical information recording medium.(182) The objective lens described in (182) wherein when recording orreproducing information for the second optical information recordingmedium, the following expressions are satisfied under the assumptionthat the intermediate optical surface area is formed within a range fromNAH mm to NAL mm in terms of the distance from an optical axis, when anecessary numerical aperture is represented by NA₂ and a focal length ofthe objective lens is represented by f₂.

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (100)

(NA ₂−0.20)f ₂ ≦NAL≦(NA ₂−0.04)f ₂  (101)

(183) The objective lens described in (183) wherein when recording orreproducing information for the first and second optical informationrecording media, light fluxes relating to the same light sourcewavelength are used, while, when recording or reproducing informationfor the first optical information recording medium, the light fluxpassing through the intermediate optical surface area is made to haveunder spherical aberration.(184) The objective lens described in (184) wherein when recording orreproducing information for the first and second optical informationrecording media, light fluxes relating to the light source wavelengthswhich are different each other are used, while, when recording orreproducing information for the first optical information recordingmedium, the light flux passing through the intermediate optical surfacearea is made to have over spherical aberration.(185) The objective lens described in (185) wherein the optical surfacearea including the optical axis has a function to correct sphericalaberration when conducting recording or reproducing of information forthe first optical information recording medium.(186) The objective lens described in (186) wherein the optical surfacearea including the optical axis has a function to correct temperaturecharacteristics when conducting recording or reproducing of informationfor the first optical information recording medium.(187) The objective lens described in (187) wherein when recording orreproducing information for the first and second optical informationrecording media, light fluxes relating to the same light sourcewavelength are used, and the surface on at least one side is composed ofoptical surfaces of two or more kinds, and the optical surface areaincluding the optical axis has a function to correct sphericalaberration for thickness t (t₁<t<t₂) of a transparent base board.(188) The objective lens described in (188) wherein, the optical surfacearea including the optical axis makes a light flux passing through it tohave under spherical aberration, when recording or reproducinginformation for the first optical information recording medium, and tohave over spherical aberration, when recording or reproducinginformation for the second optical information recording medium.(189) The objective lens described in (189) wherein when recording orreproducing information for the second optical information recordingmedium, the following expression is satisfied under the assumption thatthe area where spherical aberration is corrected for thickness t of thetransparent base board is formed within a range of distance NAH mm fromthe optical axis, when a necessary numerical aperture is represented byNA₂ and a focal length of the objective lens is represented by f₂.

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (102)

(190) The objective lens described in (190) is related to an opticalpickup device having a light source emitting light fluxes for the firstoptical information recording medium having a t₁-thick transparent baseboard and for the second optical information recording medium having at₂-thick transparent base board (t₁<t₂) and a light-converging opticalsystem including an objective lens converging the light fluxes emittedfrom the light source on an information recording surface through thetransparent base boards of the first and second optical informationrecording media, and conducting recording and/or reproducing ofinformation for each of the optical information recording media, whereina surface on at least one side of the objective lens is constituted withat least two or more kinds of optical surface areas in the effectivediameter of the objective lens, and a diffractive section to utilizen^(th) order light on which a ring-shaped diffractive zone is formed isformed on an optical surface area that is outermost in the directionperpendicular to the optical axis, or on the surface on the other sidethrough which a light flux passing through the outermost optical surfacearea passes, and average pitch P out of the ring-shaped diffractive zonesatisfies the following expression when a focal length of the objectivelens is represented by f.

2.00×10⁻⁴ ≦Pout/(|n|·f)≦3.50×10⁻³  (103)

(191) The optical pickup device described in (191) wherein average pitchP out of the ring-shaped diffractive zone satisfies the followingexpression.

1.00×10⁻³ ≦Pout/(|n|·f)≦3.00×10⁻³  (104)

(192) The optical pickup device described in (192) wherein a divergentlight emitted from the light source enters the objective lens.(193) The optical pickup device described in (193), wherein thefollowing expression is satisfied by image forming magnification m1 ofthe objective lens in the course of conducting recording or reproductionof information for the first optical information recording medium.

−½≦m1≦− 1/7.5  (105)

(194) The optical pickup device described in (194), wherein there isprovided a distance adjustment means that adjusts a distance between thelight source and the objective lens or between the light source and aninformation recording surface of the optical information recordingmedium.(195) The optical pickup device described in (195), wherein the distanceadjustment means adjusts the distance in accordance with a wavelength ofthe light source in room temperature.(196) The optical pickup device described in (196), wherein there isprovided a temperature adjustment means that adjusts an ambienttemperature.(197) The optical pickup device described in (197), wherein the lightsource is a semiconductor laser, and the temperature adjustment meansadjusts a temperature of the semiconductor laser.(198) The optical pickup device described in (198), wherein theobjective lens is driven in terms of focusing under the state in whichthe image forming magnification is constant substantially.(199) The optical pickup device described in (199), wherein imageforming magnification m2 of the objective lens in conducting recordingor reproduction of information for the second optical informationrecording medium is nearly the same as m1.(200) The optical pickup device described in (200), wherein theoutermost optical surface area has a function to correct sphericalaberration when recording or reproducing information for the firstoptical information recording medium.(201) The optical pickup device described in (201), wherein the opticalsurface area on at least one side of the objective lens is composed ofthree or more kinds of optical surface areas arranged in the directionperpendicular to an optical axis, and when recording or reproducinginformation for the first optical information recording medium,spherical aberration given to the light flux passing through theintermediate optical surface area is made to be discontinuous to be aflare component with respect to spherical aberration of the outermostoptical surface area, and when recording or reproducing information forthe second optical information recording medium, the light sourcepassing through the intermediate optical surface area is used.(202) The optical pickup device described in (202), wherein theintermediate optical surface area has a function to correct sphericalaberration for thickness t (t₁<t<t₂) of a transparent base board.(203) The optical pickup device described in (203), wherein whenrecording or reproducing information for the first optical informationrecording medium, a light flux passing through the optical surface areaincluding mainly the optical axis and the outermost optical surface areais used, and when recording or reproducing information for the secondoptical information recording medium, a light flux passing through theoptical surface area including mainly the optical axis and theintermediate optical surface area is used.(204) The optical pickup device described in (204), wherein whenrecording or reproducing information for the second optical informationrecording medium, the following expressions are satisfied under theassumption that the intermediate optical surface area is formed within arange from NAH mm to NAL mm in terms of the distance from an opticalaxis, when a necessary numerical aperture is represented by NA₂ and afocal length of the objective lens is represented by f₂.

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (106)

(NA ₂−0.20)f ₂ ≦NAL≦(NA ₂−0.04)f ₂  (107)

(205) The optical pickup device described in (205), wherein whenrecording or reproducing information for the first and second opticalinformation recording media, light fluxes relating to the same lightsource wavelength are used, while, when recording or reproducinginformation for the first optical information recording medium, thelight flux pasting through the intermediate optical surface area is madeto have under spherical aberration.(206) The optical pickup device described in (206), wherein whenrecording or reproducing information for the first and second opticalinformation recording media, light fluxes relating to the light sourcewavelengths which are different each other are used, while, whenrecording or reproducing information for the first optical informationrecording medium, the light flux passing through the intermediateoptical surface area is made to have over spherical aberration.(207) The optical pickup device described in (207), wherein the opticalsurface area including the optical axis has a function to correctspherical aberration when conducting recording or reproducing ofinformation for the first optical information recording medium.(208) The optical pickup device described in (208), wherein the opticalsurface area including the optical axis has a function to correcttemperature characteristics when conducting recording or reproducing ofinformation for the first optical information recording medium.(209) The optical pickup device described in (209), wherein whenrecording or reproducing information for the first and second opticalinformation recording media, light fluxes relating to the same lightsource wavelength are used, and the surface on at least one side iscomposed of optical surfaces of two or more kinds, and the opticalsurface area including the optical axis has a function to correctspherical aberration for thickness t (t₁<t<t₂) of a transparent baseboard.(210) The optical pickup device described in (210), wherein whenrecording or reproducing information for the first optical informationrecording medium, the optical surface area including the optical axismakes a light flux passing through the optical surface area closer tothe optical axis to have under spherical aberration, and when recordingor reproducing information for the second optical information recordingmedium, the optical surface area closer to the optical axis makes alight flux passing through the optical surface area closer to theoptical axis to have over spherical aberration.(211) The optical pickup device described in (211), wherein whenrecording or reproducing information for the second optical informationrecording medium, the following expression is satisfied under theassumption that the intermediate optical surface area is formed within arange of distance NAH mm from an optical axis when a necessary numericalaperture is represented by NA₂ and a focal length of the objective lensis represented by f₂.

(NA ₂−0.03)f ₂ ≦NAH≦(NA ₂+0.03)f ₂  (107)

(212) The optical pickup device described in (212), wherein a change ofspherical aberration for temperature change in a light flux which haspassed the outermost optical surface area is in the following range,when λ1 represents a wavelength of the light source.

|δSA1/δT|≦0.0005λ1rms/° C.  (108)

(213) The objective lens described in (213) is represented by anobjective lens of an optical pickup device having a light sourceemitting light fluxes for the first optical information recording mediumhaving a t₁-thick transparent base board and for the second opticalinformation recording medium having a t₂-thick transparent base board(t₁<t₂) and a light-converging optical system including an objectivelens converging the light fluxes emitted from the light source on aninformation recording surface through the transparent base boards of thefirst and second optical information recording media, and conductingrecording and/or reproducing of information for each of the opticalinformation recording media, wherein a surface on at least one side ofthe objective lens is constituted with at least two or more kinds ofoptical surface areas in the effective diameter of the objective lens,and a ring-shaped diffractive zone is formed on an optical surface areathat is outermost in the direction perpendicular to the optical axis, oron an area of the surface on the other side through which a light fluxpassing through the outermost optical surface area passes, thereby, whenconducting recording or reproducing of information for the first opticalinformation recording medium, correction of temperature characteristicsfor a light flux passing through the outermost optical surface area isconducted, and a design of spherical aberration for recording orreproducing of information for the second optical information recordingmedium is conducted, on the other hand, for a light flux passing throughthe area that is inside the outer optical surface area.

In the objective lens described in (213) employing the ring-shapeddiffractive zone, temperature characteristics are corrected for thelight flux passing the outermost optical surface area when recording orreproducing information for the first optical information recordingmedium, and there is conducted a design of spherical aberration forrecording or reproducing of information of the second opticalinformation recording medium for the light flux passing through the areainside the outer optical surface area. Therefore, it is possible toconduct correction of temperature characteristics and a design ofspherical aberration, on a well-balanced basis.

(214) The objective lens described in (214) wherein the followingexpression is satisfied by image forming magnification m1 of theobjective lens in the course of conducting recording or reproduction ofinformation for the first optical information recording medium.

−½≦m1≦− 1/7.5  (109)

(215) The objective lens described in (215) wherein image formingmagnification m2 of the objective lens in conducting a recording orreproduction of information for the second optical information recordingmedium is nearly the same as m1.(216) The objective lens described in (216) wherein the optical surfacearea on at least one side of the objective lens is composed of three ormore kinds of optical surface areas arranged in the directionperpendicular to an optical axis, and the optical surface area tocorrect spherical aberration for a light flux for recording orreproducing information for the first optical information recordingmedium is arranged inside the optical surface area for recording orreproducing information for the second optical information recordingmedium.(217) The objective lens described in (217) wherein the optical surfacearea on at least one side of the objective lens is composed of three ormore kinds of optical surface areas arranged in the directionperpendicular to an optical axis, and the optical surface area tocorrect temperature characteristics for a light flux for recording orreproducing information for the first optical information recordingmedium is arranged inside the optical surface area for recording orreproducing information for the second optical information recordingmedium.(218) The objective lens described in (218) is represented by anobjective lens of an optical pickup device having therein a first lightsource with wavelength λ₁ that emits a light flux to the first opticalinformation recording medium having a t₁-thick transparent base board, asecond light source with wavelength λ₂ (λ₁<λ₂) that emits a light fluxto the second optical information recording medium having a t₂-thick(t₁<t₂) transparent base board, and a light-converging optical systemincluding an objective lens that converges light fluxes emittedrespectively from the first and second light sources on the informationrecording surface respectively through transparent base boards of thefirst and second optical information recording media, and conductsrecording and/or reproducing of information for each optical informationrecording medium, wherein a surface on at least one side of theobjective lens is constituted with at least two or more kinds of opticalsurface areas in the effective diameter of the objective lens, and aring-shaped diffractive zone is formed on an optical surface area thatis outermost in the direction perpendicular to the optical axis, or onan area of the surface on the other side through which a light fluxpassing through the outermost optical surface area passes, thereby, whenconducting recording or reproducing of information for the first opticalinformation recording medium, correction of temperature characteristicsfor a light flux passing through the outermost optical surface area isconducted, and a design of spherical aberration for recording orreproducing of information for the second optical information recordingmedium is conducted, on the other hand, for a light flux passing throughthe area that is inside the outer optical surface area.(219) The objective lens described in (219) wherein the followingexpression is satisfied by image forming magnification m1 of theobjective lens in the course of conducting recording or reproduction ofinformation for the first optical information recording medium.

−½≦m1≦− 1/7.5  (110)

(220) The objective lens described in (220) wherein image formingmagnification m2 of the objective lens in conducting recording orreproduction of information for the second optical information recordingmedium is nearly the same as m1.(221) The objective lens described in (221) wherein the optical surfacearea on at least one side of the objective lens is composed of three ormore kinds of optical surface areas arranged in the directionperpendicular to an optical axis, an optical surface area used only whenthe second light source with wavelength λ₂ is used in the intermediateoptical surface area is formed, and the optical surface area to conductcorrection of spherical aberration for the light flux from the firstlight source with wavelength λ₁ is arranged inside the intermediateoptical surface area.(222) The objective lens described in (222) wherein the optical surfacearea on at least one side of the objective lens is composed of three ormore kinds of optical surface areas arranged in the directionperpendicular to an optical axis, an optical surface area used only whenthe second light source with wavelength λ₂ is used in the intermediateoptical surface area is formed, and the optical surface area to conductcorrection of temperature characteristics for the light flux from thefirst light source with wavelength λ₁ is arranged inside theintermediate optical surface area.(223) The objective lens described in (223) wherein an optical surfacearea for the exclusive use of the light flux from the second lightsource and the outermost optical surface area are adjacent to eachother.(224) The objective lens described in (224) wherein average pitch P outof the ring-shaped diffractive zone utilizing n-th order light satisfiesthe following expression, when a focal length of the objective lens isrepresented by f.

2.00×10⁻⁴ ≦Pout/(|n|·f)≦3.5×10⁻³  (111)

(225) The objective lens described in (225) wherein spherical aberrationin light fluxes passing respectively through the outermost opticalsurface area and the intermediate optical surface area adjacent to theoutermost optical surface area is discontinuous.(226) The objective lens described in (226) wherein at least one of adiffractive section and a refraction section is arranged on theintermediate optical surface area.(227) The objective lens described in (227) which is made of plasticmaterials.(228) The optical pickup device described in (228) is represented by anoptical pickup having a light source emitting light fluxes for the firstoptical information recording medium having a t₁-thick transparent baseboard and for the second optical information recording medium having at₂-thick transparent base board (t₁<t₂) and a light-converging opticalsystem including an objective lens converging the light fluxes emittedfrom the light source on an information recording surface through thetransparent base boards of the first and second optical informationrecording media, and conducting recording and/or reproducing ofinformation for each of the optical information recording media, whereina surface on at least one side of the objective lens is constituted withat least two or more kinds of optical surface areas in the effectivediameter of the objective lens, and a ring-shaped diffractive zone isformed on an optical surface area that is outermost in the directionperpendicular to the optical axis, or on an area of the surface on theother side through which a light flux passing through the outermostoptical surface area passes, thereby, when conducting recording orreproducing of information for the first optical information recordingmedium, correction of temperature characteristics for a light fluxpassing through the outermost optical surface area is conducted, and adesign of spherical aberration for recording or reproducing ofinformation for the second optical information recording medium isconducted, on the other hand, for a light flux passing through the areathat is inside the outer optical surface area.(229) The optical pickup device described in (229) wherein the followingexpression is satisfied by image forming magnification m1 of theobjective lens in the course of conducting recording or reproduction ofinformation for the first optical information recording medium.

−½≦m1≦− 1/7.5  (112)

(230) The optical pickup device described in (230) wherein image formingmagnification m2 of the objective lens in conducting recording orreproduction of information for the second optical information recordingmedium is nearly the same as m1.(231) The optical pickup device described in (231) wherein the opticalsurface area on at least one side of the objective lens is composed ofthree or more kinds of optical surface areas arranged in the directionperpendicular to an optical axis, and the optical surface area tocorrect spherical aberration for a light flux for recording orreproducing information for the first optical information recordingmedium is arranged inside the optical surface area for recording orreproducing information for the second optical information recordingmedium.(232) The optical pickup device described in (232) wherein the opticalsurface area on at least one side of the objective lens is composed ofthree or more kinds of optical surface areas arranged in the directionperpendicular to an optical axis, and the optical surface area tocorrect temperature characteristics for a light flux for recording orreproducing information for the first optical information recordingmedium is arranged inside the optical surface area for recording orreproducing information for the second optical information recordingmedium.(233) The optical pickup device described in (233) is represented by anoptical pickup device having therein a first light source withwavelength λ₁ that emits a light flux to the first optical informationrecording medium having a t₁-thick transparent base board, a secondlight source with wavelength λ₂ (λ₁<λ₂) that emits a light flux to thesecond optical information recording medium having a t₂-thick (t₁<t₂)transparent base board, and a light-converging optical system includingan objective lens that converges light fluxes emitted respectively fromthe first and second light sources on the information recording surfacerespectively through transparent base boards of the first and secondoptical information recording media, and conducts recording and/orreproducing of information for each optical information recordingmedium, wherein a surface on at least one side of the objective lens isconstituted with at least two or more kinds of optical surface areas inthe effective diameter of the objective lens, and a ring-shapeddiffractive zone is formed on an optical surface area that is outermostin the direction perpendicular to the optical axis of the objectivelens, or on an area of the surface on the other side through which alight flux passing through the outermost optical surface area passes,thereby, when conducting recording or reproducing of information for thefirst optical information recording medium, correction of temperaturecharacteristics for a light flux passing through the outermost opticalsurface area is conducted, and a design of spherical aberration forrecording or reproducing of information for the second opticalinformation recording medium is conducted, on the other hand, for alight flux passing through the area that is inside the outer opticalsurface area.(234) The optical pickup device described in (234) wherein the followingexpression is satisfied by image forming magnification m1 of theobjective lens in the course of conducting recording or reproduction ofinformation for the first optical information recording medium.

−½≦m1≦− 1/7.5  (113)

(235) The optical pickup device described in (235) wherein image formingmagnification m2 of the objective lens in conducting recording orreproduction of information for the second optical information recordingmedium is nearly the same as m1.(236) The optical pickup device described in (236) wherein the opticalsurface area on at least one side of the objective lens is composed ofthree or more kinds of optical surface areas arranged in the directionperpendicular to an optical axis, and an optical surface area to correctspherical aberration for the light flux from the first light source withwavelength λ1 is arranged inside the optical surface area for the lightflux from the second light source with wavelength λ₂.(237) The optical pickup device described in (237) wherein the opticalsurface area on at least one side of the objective lens is composed ofthree or more kinds of optical surface areas arranged in the directionperpendicular to an optical axis, and an optical surface area to correcttemperature characteristics for the light flux from the first lightsource with wavelength λ1 is arranged inside the optical surface areafor the light flux from the second light source with wavelength λ₂.(238) The optical pickup device described in (238) wherein an opticalsurface area for the light flux from the second light source and theoutermost optical surface area are adjacent to each other.(239) The optical pickup device described in (239) wherein average pitchP out of the ring-shaped diffractive zone utilizing n^(th) order lightsatisfies the following expression, when a focal length of the objectivelens is represented by f.

2.00×10⁻⁴ ≦Pout/(|n|·f)≦3.5×10⁻³  (114)

(240) The optical pickup device described in (240) wherein sphericalaberration in the outermost optical surface area and in the opticalsurface area for the light flux from the second light source isdiscontinuous.(241) The optical pickup device described in (241) wherein at least oneof a diffractive section and a refraction section is arranged on theoptical surface area for the exclusive use of the light flux from thesecond light source.(242) The optical pickup device described in (242), wherein a change ofspherical aberration for temperature change in a light flux which haspassed the outermost optical surface area is in the following range,when λ1 represents a wavelength of the light source at the roomtemperature.

|δSA1/∂T|≦0.0005λ1rm's/° C.  (115)

(243) The optical pickup device described in (243) wherein the objectivelens is made of plastic materials.(244) The objective lens described in (244) wherein the expression of|n|=1 holds for the diffraction number of order represented by n.(245) The optical pickup device described in (245) wherein theexpression of |n|=1 holds for the diffraction number of orderrepresented by n.

The structure to attain the second object is explained hereinafter.

(2-1) The objective lens for an optical pickup device described in (2-1)is represented by an objective lens for an optical pickup device havingthereina first light source with wavelength λ1 for conducting recording orreproducing for information by radiating a light flux to the firstoptical information recording medium having transparent base boardthickness t₁, a second light source with wavelength λ2 (λ1<λ2) forconducting recording or reproducing for information by radiating a lightflux to the second optical information recording medium havingtransparent base board thickness t₂ (t₁<t₂) and a light-convergingoptical system including the objective lens that converges light fluxesemitted from the first and the second light sources on the informationrecording surface through transparent base boards of the first and thesecond optical information recording media, wherein the objective lensis made of a uniform optical material, a value of refractive indexchange (hereinafter referred to as refractive index temperaturedependency) dn/dT for the temperature change of the optical materials isexpressed by the following expression under the conditions of theaforesaid light source wavelength and the room temperature environment,

|dn/dT|≦10.0×10⁻⁶(/° C.)  (117)

the objective lens is formed in a way that each of at least twooptically functional surfaces arranged in the direction intersecting anoptical axis has a different optical function, and a light flux passingthrough at least the outermost optically functional surface is used onlyfor recording or reproducing of information for the first opticalinformation recording medium.

By using a material having small temperature dependency for theobjective lens, it is possible to make a change in spherical aberrationcaused by temperature changes to be small. Therefore, when the objectivelens is composed of a refracting interface, it is easy to maketemperature characteristics to be compatible with wavelengthcharacteristics, because wavelength dependency is originally small.Further, even in the case of constituting the objective lens with adiffraction surface, a pitch of the ring-shaped diffractive zone is notrequired to be small, because temperature characteristics are improvedeven when the effectiveness of diffraction is not enhanced, which isdifferent from a conventional objective lens. In addition, when anobjective lens is provided with a plurality of optically functionalsurfaces each being designed properly, it is possible to attain a spotdiameter which is needed for optical information recording media eachhaving a different transparent base board thickness, and thereby toconduct recording or reproducing for each optical information recordingmedium. In this case, the optically functional surface that makes theoptical function to be different includes optical surfaces each beingcompletely different from others such as a refracting interface and asurface of a diffractive structure, and optical surfaces in the sametype, for example, aspheric surfaces each having a different functionwhich are formed by different aspherical coefficients, and opticalsurfaces each having a diffractive structure based on a differentdesign.

(2-2) In the objective lens for an optical pickup device described in(2-2), when each optically functional surface is formed to have a stepat a boundary section, it is easy to manipulate an amount ofdiscontinuousness of spherical aberration, and for example, a main spotlight can be separated greatly from a flare light on a recording surfaceof an optical information recording medium.(2-3) In the objective lens for an optical pickup device described in(2-3) represents an example to constitute an objective lens only with arefracting interface. When a necessary numerical aperture of the firstoptical information recording medium is greater than that of the secondoptical information recording medium, it is possible to form a sportdiameter required for the second optical information recording medium,by utilizing the first optical information recording medium and thesecond optical information recording medium in common at an area nearthe optical axis and by designing so that an intermediate opticallyfunctional surface is used for the second optical information recordingmedium. When the first optical information recording medium is used, alight flux passing through the intermediate optically functional surfaceturns out to be a flare light, but if the spherical aberrationcorrection for the first optical information recording medium is made onthe outermost optically functional surface, the required spot diametercan be formed on the first optical information recording medium.(2-4) In the objective lens for an optical pickup device described in(2-4), it is preferable for correction of spherical aberration on thesecond optical information recording medium that the step on theboundary section farther from the optical axis is greater than that onthe boundary section closer to the optical axis on the intermediateoptically functional surface.(2-5) As in the objective lens for an optical pickup device described in(2-5), if spherical aberration for recording or reproducing ofinformation for the first optical information recording medium iscorrected to 0.04 λ₁ rms or less for the innermost optically functionalsurface and the outermost optically functional surface, and if sphericalaberration is corrected to be smallest for the optical informationrecording medium with transparent base board thickness t_(c)(t₁<t_(c)<t₂), an amount of spot light for each spot light can beenhanced, which is more preferable from the viewpoint of the utilityfactor of using light.(2-6) In the objective lens for an optical pickup device described in(2-6), the objective lens has at least two optically functional surfacesand at least one optically functional surface has a diffractivestructure, the optically functional surface closest to the optical axisis designed so that spherical aberration in the course of conductingrecording or reproducing of information for the first and second opticalinformation recording media may be corrected by the use of a light fluxpassing through the optically functional surface closest to the opticalaxis, and on the outermost optically functional surface, sphericalaberration in the first optical information recording medium iscorrected, and over spherical aberration is generated on the secondoptical information recording medium, therefore, each opticallyfunctional surface is made to correspond to a plurality of opticalinformation recording media each having a different transparent baseboard thickness, thus recording or reproducing of information can beconducted properly for these optical information recording media.(2-7) In the objective lens for an optical pickup device described in(2-7), a light flux passing through each optically functional surfacepasses through the diffractive structure with either surface of theobjective lens (namely, the surface closer to the light source or thesurface closer to the optical information recording medium), while, adiffraction pitch of the diffractive structure of the outermostoptically functional surface is in a range from 5 μm to 40 μm, therebyit is possible to control a decline of diffraction efficiency whilekeeping the productivity for the objective lens.(2-8) In the objective lens for an optical pickup device described in(2-8), over spherical aberration generated in the course of conductingrecording or reproducing of information for the second opticalinformation recording medium is increased toward the periphery from theoptical axis, and therefore, recording or reproducing of information canbe conducted properly for a plurality of optical information recordingmedia each having a different transparent base board thickness.(2-9) In the objective lens for an optical pickup device described in(2-9), spherical aberration generated in the course of conductingrecording or reproducing of information for the second opticalinformation recording medium is discontinuous on the boundary section ofthe optically functional surface, and an amount of discontinuousness ofspherical aberration is in a range from 10 μm to 30 μm, thus, if theamount of discontinuousness of spherical aberration is not less than 10μm, it is possible to control that a flare approaches the main spot,while if the amount of discontinuousness of spherical aberration is notmore than 30 μm, it is possible to improve temperature characteristicssatisfactorily.(2-10) In the objective lens for an optical pickup device described in(2-10), it is possible to keep the diffraction efficiency to be highbecause recording or reproducing of information is conducted by the useof the diffracted light in the same order on the innermost opticallyfunctional surface for both the first optical information recordingmedium and the second optical information recording medium.(2-11) In the objective lens for an optical pickup device described in(2-11), it is possible to lower a light amount for flare light bylowering efficiency of diffracted light generated by the diffractivestructure of the outside optically functional surface, for example, andthereby to conduct recording or reproducing of information properly fora plurality of optical information recording media each having adifferent transparent base board thickness, because diffraction ordern_(ot) of the diffracted light having the highest intensity generated atthe diffractive structure on the outside optically functional surfaceand diffraction order n_(in) of the diffracted light having the highestintensity generated at the diffractive structure on the inside opticallyfunctional surface satisfy the following expression when conductingrecording or reproducing of information for the first opticalinformation recording medium.

|n_(ot)|≧|n_(in)|  (3)

(2-12) In the objective lens for an optical pickup device described in(2-12), with regard to the diffractive structure, a serrated ring-shapeddiffractive zone is formed, and a design basis wavelength of thering-shaped diffractive zone formed on the outside optically functionalsurface is different from that of the ring-shaped diffractive zoneformed on the inside optically functional surface, and therefore, it ispreferable, from the viewpoint of balance of an amount of light, toemploy the design basis wavelength that is between λ₁ and λ₂ on theinside optically functional surface used for both the first and secondoptical information recording media from a viewpoint of diffractionefficiency, and it is advantageous in terms of an amount of light tomake the design basis wavelength to be close to λ₁, because the outsideoptically functional surface is utilized only for the first opticalinformation recording medium.(2-13) In the objective lens for an optical pickup device described in(2-13), the objective lens has at least three optically functionalsurfaces wherein the innermost optically functional surface is composedonly of a refracting interface and the intermediate optically functionalsurface has a diffractive structure, and when a light flux used forrecording or reproducing of information for the first and second opticalinformation recording media passes through the intermediate opticallyfunctional surface, it is possible to conduct recording or reproducingof information properly for a plurality of optical information recordingmedia each having a different transparent base board thickness.(2-14) In the objective lens for an optical pickup device described in(2-14), recording or reproducing of information for the first opticalinformation recording medium can be conducted properly, because aserrated ring-shaped diffractive zone is formed on the outermostoptically functional surface and a design basis wavelength λ₀ satisfiesthe following expression.

0.9λ₁≦λ₀≦1.1λ₁

(2-15) In the objective lens for an optical pickup device described in(2-15), the outermost optically functional surface can also be composedonly of a refracting interface.(2-16) In the objective lens for an optical pickup device described in(2-16), image forming magnification m1 of the objective lens forrecording or reproducing of information for the first opticalinformation recording medium can satisfy the following expression.

−¼≦m1≦⅛  (119)

In this case, if image forming magnification m1 is not less than thelower limit, image height characteristics are excellent, while if it isnot more than the upper limit, a working distance of the objective lenscan be secured, which is preferable.(2-17) In the objective lens for an optical pickup device described in(2-17), image forming magnification m2 of the objective lens forrecording or reproducing of information for the second opticalinformation recording medium can satisfy the following expression.

0.98 m1≦m2≦1.02m1  (120)

When m1 is different from m2 in this case, and when an image formingposition on the first optical information recording medium and that onthe second optical information recording medium are made to be almostcommon for the objective lens, a light emission point is shifted, andthere is the possibility of complicated optical system includingpreparation of two sensors for signal detection. Namely, if theexpression (120) is satisfied, signal detection in the course ofrecording and reproducing for each of the first optical informationrecording medium and the second optical information recording medium canbe conducted by a single sensor.(2-18) In the objective lens for an optical pickup device described in(2-18), if an aperture-stop in the case of conducting recording orreproducing of information for the first optical information recordingmedium is the same as that in the case of conducting recording orreproducing of information for the second optical information recordingmedium, it is possible to simplify the construction of the opticalpickup device.(2-19) In the objective lens for an optical pickup device described in(2-19), if necessary numerical aperture NA1 in the case of conductingrecording or reproducing of information for the first opticalinformation recording medium satisfies the following expression, it ispossible to in conduct high density information recording or highdensity information reproducing.

NA1≧0.60  (121)

(2-20) In the objective lens for an optical pickup device described in(2-20), if wavelength λ1 of the first light source is not more than 670nm, a high density optical information recording medium such as DVDrepresenting the first optical information recording medium can be used.(2-21) In the objective lens for an optical pickup device described in(2-21), when the optical material is represented by optical glass anddispersion value νd is greater than 50, a change of refractive indexcaused by temperature changes is less and axial chromatic aberration canbe made excellent, which is preferable. Incidentally, the objective lensdescribed in either one of the aforesaid structures 1-21 has the sameaction and effect as those stated above, even in the invention includingthe optical pickup device employing that objective lens, the objectivelens wherein a plurality of optical elements are cemented, and theoptical pickup device employing the aforesaid objective lens all will beexplained later.(2-22) The optical pickup device described in (2-22) is represented byan optical pickup device having therein a first light source withwavelength λ₁ arranged to conduct recording or reproducing ofinformation by radiating a light flux to the first optical informationrecording medium having transparent base board thickness t₁, a secondlight source with wavelength λ₂ (λ₁<λ₂) arranged to conduct recording orreproducing of information by radiating a light flux to the secondoptical information recording medium having transparent base boardthickness t₂ (t₁<t₂), and a light-converging optical system including anobjective lens that converges light fluxes radiated respectively fromthe first and second light sources on information recording surfacesthrough transparent base boards respectively of the first and secondoptical information recording media, wherein the objective lens is madeof uniform optical material, refractive index change dn/dT of theoptical material for temperature changes satisfies the followingexpression under the conditions of the wavelength of the light sourceand the temperature environment for room temperature,

|dn/dT|≦10.0×10⁻⁶(/° C.)  (127)

the objective lens is formed to make an optical action to be differenton each of at least two optically functional surfaces arranged in thedirection intersecting an optical axis, and a light flux passing throughat least the outermost optically functional surface is used only forrecording or reproducing of information for the first opticalinformation recording medium. Action and effect of the invention statedabove are the same as those of the invention described in (2-1).(2-23) In the optical pickup device described in (2-23), each opticallyfunctional surface mentioned above is formed to have a step at theboundary section. Action and effect of the invention stated above arethe same as those of the invention described in (2-2).(2-24) In the optical pickup device described in (2-24), the objectivelens is composed only of a refracting interface, at least threeoptically functional surfaces are formed, a light flux passing throughthe innermost optically functional surface is used for conductingrecording or reproducing of information for the first and second opticalinformation recording media, a light flux passing through theintermediate optically functional surface is used for conductingrecording or reproducing of information for the second opticalinformation recording medium, and a light flux passing through theoutermost optically functional surface is used for conducting recordingor reproducing of information for the first optical informationrecording medium. Action and effect of the invention stated above arethe same as those of the invention described in (2-3).(2-25) In the optical pickup device described in (2-25), a height of thestep on the boundary section that is farther from an optical axis isgreater than that on the boundary section that is closer to the opticalaxis, on the intermediate optically functional surface. Action andeffect of the invention stated above are the same as those of theinvention described in (2-4).(2-26) In the optical pickup device described in (2-26), with respect tothe innermost optically functional surface and the outermost opticallyfunctional surface, spherical aberration in the course of conductingrecording or reproducing of information for the first opticalinformation recording medium is corrected to 0.04 λ₁ rms or less, andthe intermediate optically functional surface is corrected so that itsspherical aberration for the optical information recording medium havingtransparent base board thickness t_(c) (t₁<t_(c)<t₂) may be the minimum.Action and effect of the invention stated above are the same as those ofthe invention described in (2-5).(2-27) In the optical pickup device described in (2-27), the objectivelens has at least two optically functional surfaces, and at least one ofthem has a diffractive structure, and the optically functional surfaceclosest to the optical axis is designed to correct its sphericalaberration in the course of conducting recording or reproducing ofinformation for the first and second optical information recording mediaby using the light flux passing through the optically functional surfaceclosest to the optical axis, and on the outermost optically functionalsurface, spherical aberration for the first optical informationrecording medium is corrected, while over spherical aberration isgenerated for the second optical information recording medium. Actionand effect of the invention stated above are the same as those of theinvention described in (2-6).(2-28) In the optical pickup device described in (2-28), a light fluxpassing through each optically functional surface mentioned above passesthrough the aforesaid diffractive structure on either surface of theobjective lens, and a diffraction pitch of the diffractive structure onthe outermost optically functional surface is in a range from 5 μm to 40μm. Action and effect of the invention stated above are the same asthose of the invention described in (2-7).(2-29) In the optical pickup device described in (2-29), the overspherical aberration generated in the course of conducting recording orreproducing of information for the a second optical informationrecording medium is made to be increased gradually in the direction fromthe optical axis side toward the periphery. Action and effect of theinvention stated above are the same as those of the invention describedin (2-8).(2-30) In the optical pickup device described in (2-30), the sphericalaberration generated in the course of conducting recording orreproducing of information for the second optical information recordingmedium is discontinuous at the boundary section of the opticallyfunctional surface, and an amount of discontinuousness of the sphericalaberration is in a range from 10 μm to 30 μm. Action and effect of theinvention stated above are the same as those of the invention describedin (2-9).(2-31) In the optical pickup device described in (2-31), recording orreproducing of information is conducted by the use of the diffractedlight in the same order on the innermost optically functional surface,for the first and second optical information recording media. Action andeffect of the invention stated above are the same as those of theinvention described in (2-10).(2-32) In the optical pickup device described in (2-32), when conductingrecording or reproducing of information for the first opticalinformation recording medium, diffraction order n_(ot) of the diffractedlight having the highest intensity generated at the diffractivestructure on the outside optically functional surface and diffractionorder n_(in) of the diffracted light having the highest intensitygenerated at the diffractive structure on the inside opticallyfunctional surface satisfy the following expression.

|n_(ot)|≧|n_(in)|  (128)

Action and effect of the invention stated above are the same as those ofthe invention described in (2-11).(2-33) In the optical pickup device described in (2-33), in thediffractive structure stated above, a serrated ring-shaped diffractivezone is formed, and a design basis wavelength of the ring-shapeddiffractive zone formed on the outside optically functional surface isdifferent from that of the ring-shaped diffractive zone formed on theinside optically functional surface. Action and effect of the inventionstated above are the same as those of the invention described in (2-12).(2-34) In the optical pickup device described in (2-34), the objectivelens has at least three optically functional surfaces wherein theinnermost optically functional surface is composed only of a refractinginterface and the intermediate optically functional surface has adiffractive structure, and a light flux used for recording orreproducing of information for the first and second optical informationrecording media passes through the intermediate optically functionalsurface. Action and effect of the invention stated above are the same asthose of the invention described in (2-13).(2-35) In the optical pickup device described in (2-35), a serratedring-shaped diffractive zone is formed on the outermost opticallyfunctional surface, and design basis wavelength λ₀ of the outermostoptically functional surface satisfies 9λ₁≦λ₀≦1.1λ₁. Action and effectof the invention stated above are the same as those of the inventiondescribed in (2-14).(2-36) In the optical pickup device described in (2-36), the outermostoptically functional surface is composed only of a refracting interface.Action and effect of the invention stated above are the same as those ofthe invention described in (2-15).(2-37) In the optical pickup device described in (2-37), image formingmagnification m1 of the objective lens for conducting recording orreproducing of information for the first optical information recordingmedium satisfies the following expression.

−¼≦m1≦⅛  (129)

Action and effect of the invention stated above are the same as those ofthe invention described in (2-16).(2-38) In the optical pickup device described in (2-38), image formingmagnification m2 of the objective lens for conducting recording orreproducing of information for the second optical information recordingmedium satisfies the following expression.

0.98m1≦m2≦1.02m1  (130)

Action and effect of the invention stated above are the same as those ofthe invention described in (2-17).(2-39) In the optical pickup device described in (2-39) an aperture-stopin the case of conducting recording or reproducing of information forthe first optical information recording medium is the same as that inthe case of conducting recording or reproducing of information for thesecond optical information recording medium. Action and effect of theinvention stated above are the same as those of the invention describedin (2-18).(2-40) In the optical pickup device described in (2-40), necessarynumerical aperture NA1 in the case of conducting recording orreproducing of information for the first optical information recordingmedium satisfies the following expression.

NA1≦0.60  (131)

Action and effect of the invention stated above are the same as those ofthe invention described in (2-19).(2-41) In the optical pickup device described in (2-41), wavelength λ1of the first light source is not more than 670 nm. Action and effect ofthe invention stated above are the same as those of the inventiondescribed in (2-20).(2-42) In the optical pickup device described in (2-42), the opticalmaterial is represented by optical glass and dispersion value νd isgreater than 50. Action and effect of the invention stated above are thesame as those of the invention described in (2-21).(2-43) The objective lens of an optical pickup device described in(2-43) is represented by an objective lens of an optical pickup devicehaving therein a first light source with wavelength λ₁ arranged toconduct recording or reproducing of information by radiating a lightflux to the first optical information recording medium havingtransparent base board thickness t₁, a second light source with awavelength λ₂ (λ₁<λ₂) arranged to conduct recording or reproducing ofinformation by radiating a light flux to the second optical informationrecording medium having transparent base board thickness t₂ (t₁<t₂), anda light-converging optical system including an objective lens thatconverges light fluxes radiated respectively from the first and secondlight sources on information recording surfaces through transparent baseboards respectively of the first and second optical informationrecording media, wherein the objective lens is a cemented lens formed bycementing plural optical elements made of at least two kinds of opticalmaterials, a value of refractive index change dn/dT of the opticalmaterial used for the optical element having stronger power componentamong the plural optical elements for temperature changes satisfies thefollowing expression,

|dn/dT|≦10.0×10⁻⁶(/° C.).  (127)

and the objective lens is formed to make an optical action to bedifferent on each of at least two optically functional surfaces arrangedin the direction intersecting an optical axis, and a light flux passingthrough at least the outermost optically functional surface is used onlyfor recording or reproducing of information for the first opticalinformation recording medium, and therefore, it is possible to conductrecording or reproducing of information properly for plural opticalinformation recording media each having a different transparent baseboard thickness by forming the objective lens by combining a materialwhose refractive index change for temperature changes is small andanother material that is different from the previous material. Whenforming the objective lens by cementing optical elements, if temperaturedependency of the material for the lens having stronger power is made tobe lower, it is possible to make the total temperature dependency of thecemented objective lens to be low.(2-44) In the objective lens of an optical pickup device described in(2-44), at least one of optical elements other than those havingstronger power components among the aforesaid plural optical elements ismade of plastic material, and therefore, a different opticallyfunctional surface can easily be constituted because of characteristicsthat forming is easy, which is an advantage.(2-45) In the objective lens of an optical pickup device described in(2-45), a plurality of optically functional surfaces are formed on anoptical surface of the optical element that is made of plastic material,thus, an objective lens which can be easily manufactured is provided.(2-46) In the objective lens of an optical pickup device described in(2-46), each optically functional surface mentioned above is formed tohave a step at the boundary section. Action and effect of the inventionstated above are the same as those of the invention described in (2-2).(2-47) In the objective lens of an optical pickup device described in(2-47), the objective lens is composed only of a refracting interface,at least three optically functional surfaces are formed, a light fluxpassing through the innermost optically functional surface is used forconducting recording or reproducing of information for the first andsecond optical information recording media, a light flux passing throughthe intermediate optically functional surface is used for conductingrecording or reproducing of information for the second opticalinformation recording medium, and a light flux passing through theoutermost optically functional surface is used for conducting recordingor reproducing of information for the first optical informationrecording medium. Action and effect of the invention stated above arethe same as those of the invention described in (2-3).(2-48) In the objective lens of an optical pickup device described in(2-48), a height of the step on the boundary section that is fartherfrom an optical axis is greater than that on the boundary section thatis closer to the optical axis, on the intermediate optically functionalsurface. Action and effect of the invention stated above are the same asthose of the invention described in (2-4).(2-49) In the objective lens of an optical pickup device described in(2-49), with respect to the innermost optically functional surface andthe outermost optically functional surface, spherical aberration in thecourse of conducting recording or reproducing of information for thefirst optical information recording medium is corrected to 0.04 λ₁ rmsor less, and the intermediate optically functional surface is correctedso that its spherical aberration for the optical information recordingmedium having transparent base board thickness t_(c) (t₁<t_(c)<t₂) maybe the minimum. Action and effect of the invention stated above are thesame as those of the invention described in (2-5).(2-50) In the objective lens of an optical pickup device described in(2-50), the objective lens has at least two optically functionalsurfaces, and at least one of them has a diffractive structure, and theoptically functional surface closest to the optical axis is designed tocorrect its spherical aberration in the course of conducting recordingor reproducing of information for the first and second opticalinformation recording media by using the light flux passing through theoptically functional surface closest to the optical axis, and on theoutermost optically functional surface, spherical aberration for thefirst optical information recording medium is corrected, while overspherical aberration is generated for the second optical informationrecording medium. Action and effect of the invention stated above arethe same as those of the invention described in (2-6).(2-51) In the objective lens of an optical pickup device described in(2-51), a light flux passing through each optically functional surfacementioned above passes through the aforesaid diffractive structure oneither surface of the objective lens, and a diffraction pitch of thediffractive structure on the outermost optically functional surface isin a range from 5 μm to 40 μm. Action and effect of the invention statedabove are the same as those of the invention described in (2-7).(2-52) In the objective lens of an optical pickup device described in(2-52), the over spherical aberration generated in the course ofconducting recording or reproducing of information for the secondoptical information recording medium is made to be increased graduallyin the direction from the optical axis side toward the periphery. Actionand effect of the invention stated above are the same as those of theinvention described in (2-8).(2-53) In the objective lens of an optical pickup device described in(2-53), the spherical aberration generated in the course of conductingrecording or reproducing of information for the second opticalinformation recording medium is discontinuous at the boundary section ofthe optically functional surface, and an amount of discontinuousness ofthe spherical aberration is in a range from 10 μm to 30 μm. Action andeffect of the invention stated above are the same as those of theinvention described in (2-9).(2-54) In the objective lens of an optical pickup device described in(2-54), recording or reproducing of information is conducted by the useof the diffracted light in the same order on the innermost opticallyfunctional surface, for the first and second optical informationrecording media. Action and effect of the invention stated above are thesame as those of the invention described in (2-10).(2-55) In the objective lens of an optical pickup device described in(2-55), when conducting recording or reproducing of information for thefirst optical information recording medium, diffraction order not of thediffracted light having the highest intensity generated at thediffractive structure on the outside optically functional surface anddiffraction order n_(in) of the diffracted light having the highestintensity generated at the diffractive structure on the inside opticallyfunctional surface satisfy the following expression.

|n_(ot)|≧|n_(in)|  (128)

Action and effect of the invention stated above are the same as those ofthe invention described in (2-11).(2-56) In the objective lens of an optical pickup device described in(2-56), in the diffractive structure stated above, a serratedring-shaped diffractive zone is formed, and a design basis wavelength ofthe ring-shaped diffractive zone formed on the outside opticallyfunctional surface is different from that of the ring-shaped diffractivezone formed on the inside optically functional surface. Action andeffect of the invention stated above are the same as those of theinvention described in (2-12).(2-57) In the objective lens for an optical pickup device described in(2-57), the objective lens has at least three optically functionalsurfaces wherein the innermost optically functional surface is composedonly of a refracting interface and the intermediate optically functionalsurface has a diffractive structure, and a light flux used for recordingor reproducing of information for the first and second opticalinformation recording media passes through the intermediate opticallyfunctional surface. Action and effect of the invention stated above arethe same as those of the invention described in (2-13).(2-58) In the objective lens of an optical pickup device described in(2-58), a serrated ring-shaped diffractive zone is formed on theoutermost optically functional surface, and design basis wavelength λ₀of the outermost optically functional surface satisfies 9λ₁≦λ₀≦1.1λ₁.Action and effect of the invention stated above are the same as those ofthe invention described in (2-14).(2-59) In the objective lens of an optical pickup device described in(2-59), the outermost optically functional surface is composed only of arefracting interface. Action and effect of the invention stated aboveare the same as those of the invention described in (2-15).(2-60) In the objective lens of an optical pickup device described in(2-60), image forming magnification m1 of the objective lens forconducting recording or reproducing of information for the first opticalinformation recording medium satisfies the following expression.

−¼≦m1≦⅛  (129)

Action and effect of the invention stated above are the same as those ofthe invention described in (2-16).(2-61) In the objective lens of an optical pickup device described in(2-61), image forming magnification m2 of the objective lens forconducting recording or reproducing of information for the secondoptical information recording medium satisfies the following expression.

0.98 m1≦m2≦1.02m1  (130)

Action and effect of the invention stated above are the same as those ofthe invention described in (2-17).(2-62) In the objective lens of an optical pickup device described in(2-62), an aperture-stop in the case of conducting recording orreproducing of information for the first optical information recordingmedium is the same as that in the case of conducting recording orreproducing of information for the second optical information recordingmedium. Action and effect of the invention stated above are the same asthose of the invention described in (2-18).(2-63) In the objective lens of an optical pickup device described in(2-63), necessary numerical aperture NA1 in the case of conductingrecording or reproducing of information for the first opticalinformation recording medium satisfies the following expression.

NA1≧0.60  (131)

Action and effect of the invention stated above are the same as those ofthe invention described in (2-19).(2-64) In the objective lens of an optical pickup device described in(2-64), wavelength λ1 of the first light source is not more than 670 nm.Action and effect of the invention stated above are the same as those ofthe invention described in (2-20).(2-65) In the objective lens of an optical pickup device described in(2-65), the optical material is represented by optical glass anddispersion value νd is greater than 50. Action and effect of theinvention stated above are the same as those of the invention describedin (2-21).(2-66) The optical pickup device described in (2-66) is represented byan optical pickup device having therein a first light source withwavelength λ₁ arranged to conduct recording or reproducing ofinformation by radiating a light flux to the first optical informationrecording medium having transparent base board thickness t₁, a secondlight source with wavelength λ₂ (λ₁<λ₂) arranged to conduct recording orreproducing of information by radiating a light flux to the secondoptical information recording medium having transparent base boardthickness t₂ (t₁<t₂) and a light-converging optical system including anobjective lens that converges light fluxes radiated respectively fromthe first and second light sources on information recording surfacesthrough transparent base boards respectively of the first and secondoptical information recording media, wherein the objective lens is madeof uniform optical material, refractive index change dn/dT of theoptical material for temperature changes satisfies the followingexpression under the conditions of the wavelength of the light sourceand the temperature environment for room temperature,

|dn/dT|≦10.0×10⁻⁶(/° C.)  (127)

the objective lens is formed to make an optical action to be differenton each of at least two optically functional surfaces arranged in thedirection intersecting an optical axis, and a light flux passing throughat least the outermost optically functional surface is used only forrecording or reproducing of information for the first opticalinformation recording medium. Action and effect of the invention statedabove are the same as those of the invention described in (2-43).(2-67) In the optical pickup device described in (2-67), at least one ofoptical elements other than the optical element having stronger powercomponent among the aforesaid plural optical elements is made of aplastic material. Action and effect of the invention stated above arethe same as those of the invention described in (2-44).(2-68) In the optical pickup device described in (2-68), a plurality ofoptically functional surfaces are formed on an optical surface of theoptical element that is made of plastic material. Action and effect ofthe invention stated above are the same as those of the inventiondescribed in (2-45).(2-69) In the optical pickup device described in 2-69), each opticallyfunctional surface mentioned above is formed to have a step at theboundary section. Action and effect of the invention stated above arethe same as those of the invention described in (2-2).(2-70) In the optical pickup device described in (2-70), the objectivelens is composed only of a refracting interface, at least threeoptically functional surfaces are formed, a light flux passing throughthe innermost optically functional surface is used for conductingrecording or reproducing of information for the first and second opticalinformation recording media, a light flux passing through theintermediate optically functional surface is used for conductingrecording or reproducing of information for the second opticalinformation recording medium, and a light flux passing through theoutermost optically functional surface is used for conducting recordingor reproducing of information for the first optical informationrecording medium. Action and effect of the invention stated above arethe same as those of the invention described in (2-3).(2-71) In the optical pickup device described in (2-71), a height of thestep on the boundary section that is farther from an optical axis isgreater than that on the boundary section that is closer to the opticalaxis, on the intermediate optically functional surface. Action andeffect of the invention stated above are the same as those of theinvention described in (2-4).(2-72) In the optical pickup device described in (2-72), with respect tothe innermost optically functional surface and the outermost opticallyfunctional surface, spherical aberration in the course of conductingrecording or reproducing of information for the first opticalinformation recording medium is corrected to 0.04 λ₁ rms or less, andthe intermediate optically functional surface is corrected so that itsspherical aberration for the optical information recording medium havingtransparent base board thickness t_(c) (t₁<t_(c)<t₂) may be the minimum.Action and effect of the invention stated above are the same as those ofthe invention described in (2-5).(2-73) In the optical pickup device described in (2-73), the objectivelens has at least two optically functional surfaces, and at least one ofthem has a diffractive structure, and the optically functional surfaceclosest to the optical axis is designed to correct its sphericalaberration in the course of conducting recording or reproducing ofinformation for the first and second optical information recording mediaby using the light flux passing through the optically functional surfaceclosest to the optical axis, and on the outermost optically functionalsurface, spherical aberration for the first optical informationrecording medium is corrected, while over spherical aberration isgenerated for the second optical information recording medium. Actionand effect of the invention stated above are the same as those of theinvention described in (2-6).(2-74) In the optical pickup device described in (2-74), a light fluxpassing through each optically functional surface mentioned above passesthrough the aforesaid diffractive structure on either surface of theobjective lens, and a diffraction pitch of the diffractive structure onthe outermost optically functional surface is in a range from 5 μm to 40μm. Action and effect of the invention stated above are the same asthose of the invention described in (2-7).(2-75) In the optical pickup device described in (2-75), the overspherical aberration generated in the course of conducting recording orreproducing of information for the second optical information recordingmedium is made to be increased gradually in the direction from theoptical axis side toward the periphery. Action and effect of theinvention stated above are the same as those of the invention describedin (2-8).(2-76) In the optical pickup device described in (2-76), the sphericalaberration generated in the course of conducting recording orreproducing of information for the second optical information recordingmedium is discontinuous at the boundary section of the opticallyfunctional surface, and an amount of discontinuousness of the sphericalaberration is in a range from 10 μm to 30 μm. Action and effect of theinvention stated above are the same as those of the invention describedin (2-9).(2-77) In the optical pickup device described in (2-77), recording orreproducing of information is conducted by the use of the diffractedlight in the same order on the innermost optically functional surface,for the first and second optical information recording media. Action andeffect of the invention stated above are the same as those of theinvention described in (2-10).(2-78) In the optical pickup device described in (2-76), when conductingrecording or reproducing of information for the first opticalinformation recording medium, diffraction order n_(ot) of the diffractedlight having the highest intensity generated at the diffractivestructure on the outside optically functional surface and diffractionorder n_(in) of the diffracted light having the highest intensitygenerated at the diffractive structure on the inside opticallyfunctional surface satisfy the following expression.

|n_(ot)|≧|n_(in)|  (128)

Action and effect of the invention stated above are the same as those ofthe invention described in (2-11).(2-79) In the optical pickup device described in (2-79), in thediffractive structure stated above, a serrated ring-shaped diffractivezone is formed, and a design basis wavelength of the ring-shapeddiffractive zone formed on the outside optically functional surface isdifferent from that of the ring-shaped diffractive zone formed on theinside optically functional surface. Action and effect of the inventionstated above are the same as those of the invention described in (2-12).(2-80) In the optical pickup device described in (2-80), the objectivelens has at least three optically functional surfaces wherein theinnermost optically functional surface is composed only of a refractinginterface and the intermediate optically functional surface has adiffractive structure, and a light flux used for recording orreproducing of information for the first and second optical informationrecording media passes through the intermediate optically functionalsurface. Action and effect of the invention stated above are the same asthose of the invention described in (2-13).(2-81) In the optical pickup device described in (2-81), a serratedring-shaped diffractive zone is formed on the outermost opticallyfunctional surface, and design basis wavelength λ₀ of the outermostoptically functional surface satisfies 9λ₁≦λ₀≦1.1λ₁. Action and effectof the invention stated above are the same as those of the inventiondescribed in (2-14).(2-82) In the optical pickup device described in (2-82), the outermostoptically functional surface is composed only of a refracting interface.Action and effect of the invention stated above are the same as those ofthe invention described in (2-15).(2-83) In the optical pickup device described in (2-83), image formingmagnification m1 of the objective lens for conducting recording orreproducing of information for the first optical information recordingmedium satisfies the following expression.

−¼≦m1≦⅛  (129)

Action and effect of the invention stated above are the same as those ofthe invention described in (2-16).(2-84) In the optical pickup device described in (2-84), image formingmagnification m2 of the objective lens for conducting recording orreproducing of information for the second optical information recordingmedium satisfies the following expression.

0.98m1≦m2≦1.02m1  (130)

Action and effect of the invention stated above are the same as those ofthe invention described in (2-17).(2-85) In the optical pickup device described in (2-85), anaperture-stop in the case of conducting recording or reproducing ofinformation for the first optical information recording medium is thesame as that in the case of conducting recording or reproducing ofinformation for the second optical information recording medium. Actionand effect of the invention stated above are the same as those of theinvention described in (2-18).(2-86) In the optical pickup device described in (2-86), necessarynumerical aperture NA1 in the case of conducting recording orreproducing of information for the first optical information recordingmedium satisfies the following expression.

NA1≧0.60  (131)

Action and effect of the invention stated above are the same as those ofthe invention described in (2-19).(2-87) In the optical pickup device described in (2-87), wavelength λ1of the first light source is not more than 670 Action and effect of theinvention stated above are the same as those of the invention describedin (2-20).(2-88) In the optical pickup device described in (2-88), the opticalmaterial is represented by optical glass and dispersion value νd isgreater than 50. Action and effect of the invention stated above are thesame as those of the invention described in (2-21).(2-89) In the optical pickup device described in (2-89), with respect toan objective lens of the optical pickup device having therein a firstlight source with wavelength λ₁ arranged to conduct recording orreproducing of information by radiating a light flux to the firstoptical information recording medium having transparent base boardthickness t₁, a second light source with wavelength λ₂ (λ₁<λ₂) arrangedto conduct recording or reproducing of information by radiating a lightflux to the second optical information recording medium havingtransparent base board thickness t₂ (t₁<t₂), and a light-convergingoptical system including an objective lens that converges light fluxesradiated respectively from the first and second light sources oninformation recording surfaces through transparent base boardsrespectively of the first and second optical information recordingmedia, the objective lens is made of uniform optical material or iscomposed of cemented lenses, and refractive index change dn/dT fortemperature changes of the optical material having the strongest poweramong those used for constituting the objective lens satisfies thefollowing expression,

|dn/dT|≦10.0×10⁻⁶(/° C.)  (127)

and there is provided a restricting member which lowers transmissionfactor of ray of light or intercepts the ray of light in the course ofconducting recording or reproducing of information for the secondoptical information recording medium on at least the peripheral portionof the objective lens, and ray of light passing through at least thevicinity of an optical axis has been corrected in terms of sphericalaberration in the course of conducting recording or reproducing ofinformation for the first and second optical information recordingmedia, thus, it is possible to conduct recording or reproducing properlyfor a plurality of optical information recording media each having adifferent transparent base board thickness, by using a material havingless change in refractive index for temperature changes for theobjective lens and by restricting an amount of irradiation for thesecond optical information recording medium by the intercepting member.(2-90) In the optical pickup device described in (2-90), if there isprovided a wavelength-selecting diaphragm that transmits ray of lighthaving wavelength λ₁ emitted from the first light source and interceptsray of light emitted from the second light source, the structure can besimplified, which is preferable.(2-91) In the optical pickup device described in (2-91), at least oneside of the objective lens is entirely covered by diffractive structureor is provided with two or more optically functional surfaces, andtherefore, it is possible to conduct recording or reproducing ofinformation properly for a plurality of optical information recordingmedia each having a different transparent base board thickness.(2-92) In the optical pickup device described in (2-92), image formingmagnification m1 of the objective lens in the case of conductingrecording or reproducing of information for the first opticalinformation recording medium and image forming magnification m2 of theobjective lens in the case of conducting recording or reproducing ofinformation for the second optical information recording medium satisfythe following expression.

0.98m1≦m2≦1.02m1  (130)

In the present specification, when “an optical surface area” isexpressed with spherical aberration, if the spherical aberration comesunder either one of the following cases, it is assumed that there existoptical surface areas which are different from each other at a boundaryrepresented by h.

(a) Spherical aberration is discontinuous at h representing a boundary(FIG. 1 (a)).

(b) Though spherical aberration is continuous at h, the first orderdifferentiation is discontinuous (FIG. 1 (b)).

(c) Spherical aberration is discontinuous at h for a certain wavelength(FIG. 1 (a)).

The area which is divided under the conditions stated above and throughwhich each light flux passes is respectively regarded as “an opticalsurface area”. Therefore, when one surface of a lens is looked, if arefraction section and a diffractive section exist on the surface, thesesections are regarded as separate “optical surface areas” which aredifferent from each other at a boundary portion between the refractionsection and the diffractive section (see FIGS. 2 (a) and 2 (c)).Further, even when the diffractive section is formed on the entiresurface, when diffractive sections each designed for a different objectare mixed together, they are regarded as separate optical surface areasbased on the condition of the Item (c) above (see FIG. 2 (b)).Furthermore, even when aspheric surfaces expressed with the sameaspheric surface coefficient are formed on the surface on one side of alens, for example, when discontinuous portions are formed on the surfaceon the other side, they are assumed to be the separate optical surfaceareas.

In the present specification, “an area on the peripheral side” is oneoptical surface area of the aforesaid “optical surface area”, and itmeans the optical surface area closer to the peripheral side than theoptical surface area including an optical axis among a plurality ofoptical surface areas. Further, “an area on the peripheral side” is anarea existing on a part of either one of the following areas (a)-(f). Itis preferable that 80% or more of either one of the following areas(a)-(f) is represented by “the area on the peripheral side”, and it ispreferable that 100% of either one of the following areas (a)-(f) isrepresented by “the area on the peripheral side”. Next, areas (a)-(f)will be explained.

With regard optical disks popularized presently, there has generallybeen published a specification handbook in which wavelengths to be usedand numerical apertures of light fluxes entering the optical disks arestipulated. Evaluation of optical disks is made by an optical diskevaluating instrument on which an optical pickup device having therein alight source with a wavelength and a light-converging optical systemhaving a numerical aperture both based on the specification handbook ismounted. However, a wavelength of a light source on the optical pickupdevice provided on an actual optical disk apparatus does not alwaysfollow the specification handbook.

With regard to stipulations of the optical pickup device for measurementof CD, as an example, a wavelength is 780±10 nm and a numerical apertureis 0.45±0.01.

However, in the case of the optical pickup device provided on an actualCD player, a semiconductor laser whose oscillation wavelength at anordinary temperature is longer than 790 nm is used as a light sourcefrom the viewpoint of a laser life and cost, in an example of awavelength. With respect to the numerical aperture, on the other hand,there is also an occasion to use NA 0.43 for avoiding an influence of anerror or to use NA 0.47 for improving basic performances.

On an optical pickup device provided on a DVD player having bothfunctions for reproduction of DVD and that of CD, a light source with awavelength of 650 nm is used for reproduction of DVD, and the same lightsource is used also for reproduction of CD. In this case, a diameter ofan image forming spot of the light-converging optical system having noaberration is proportional to a wavelength, and is inverselyproportional to a numerical aperture of a light flux entering theoptical disk. Therefore, NA to obtain, under 650 nm, the image formingspot with the same diameter as that for NA 0.45 under 780 nm is 0.375,and the numerical aperture of about 0.38 is used. The basis why theoptical pickup device that does not comply with the specifications ofthe optical disk has been put to practical use is considered to be thecase that needs in the market have been changed from those in theinitial stage of development and peripheral technologies have madeprogress.

An apparatus to use both DVD and CD on an interchangeable basis includesthose in the following six types presently.

(1) An optical disk apparatus which employs an optical pickup devicehaving only a light source with a wavelength of about 655 nm to conductreproducing of DVD and reproducing of either one of CD and CD-ROM.

(2) An optical disk apparatus which employs an optical pickup devicehaving a first light source with a wavelength of about 655 nm and asecond light source with a wavelength of about 785 nm to conductreproducing of DVD, reproducing of either one of CD-R and CD-RW.

(3) An optical disk apparatus which employs an optical pickup devicehaving a first light source with a wavelength of about 655 nm and asecond light source with a wavelength of about 785 nm to conductreproducing of DVD, reproducing of either one of CD and CD-ROM andrecording/reproducing of either one of CD-R and CD-RW.

(4) An optical disk apparatus which employs an optical pickup devicehaving only a light source with a wavelength of about 655 nm to conductreproducing of DVD, recording/reproducing of either one of DVD-RAM,DVD-RW, DVD+RW, DVD-R and MMVF and reproducing of either one of CD andCD-ROM.

(5) An optical disk apparatus which employs an optical pickup devicehaving a first light source with a wavelength of about 655 nm and asecond light source with a wavelength of about 785 nm to conductreproducing of DVD, recording/reproducing of either one of DVD-RAM,DVD-RW, DVD+RW, DVD-R and MMVP and reproducing of either one of CD andCD-ROM and of either one of CD-R and CD-RW.

(6) An optical disk apparatus which employs an optical pickup devicehaving a first light source with a wavelength of about 655 nm and asecond light source with a wavelength of about 785 nm to conductreproducing of DVD, recording/reproducing of either one of DVD-RAN,DVD-RW, DVD+RW, DVD-R and MMVF, reproducing of either one of CD andCD-ROM and recording/reproducing of either one of CD-R and CD-RW.

Since the numerical aperture necessary for recording and reproducing foreach type of disk is different from others in each optical diskapparatus, the area on the peripheral side mentioned in the inventionvaries.

Therefore, the area on the peripheral side is determined as follows, inaccordance with a type of the optical disk apparatus.

(a) The area on the peripheral side of the objective lens in theapparatus of the aforesaid Item (1) is an area where the numericalaperture is 0.38 based on the maximum numerical aperture (usually,0.6-0.63) for the light flux emitted from the first light source toenter the optical disk.

(b) The area on the peripheral side of the objective lens in theapparatus of the aforesaid Item (2) is an area where the numericalaperture for the light flux emitted from the second light source toenter the optical disk is 0.45 based on the numerical aperture (usually,0.6-0.63) for the light flux emitted from the first light source toenter the optical disk.

(c) The area on the peripheral side of the objective lens in theapparatus of the aforesaid Item (3) is an area where the numericalaperture for the light flux emitted from the second light source toenter the optical disk is 0.50 based on the maximum numerical aperture(usually, 0.6-0.63) for the light flux emitted from the first lightsource to enter the optical disk.

(d) The area on the peripheral side of the objective lens in theapparatus of the aforesaid Item (4) is an area where the numericalaperture is 0.38 based on the maximum numerical aperture (usually,0.6-0.65) for the light flux emitted from the first light source toenter the optical disk.

(e) The area on the peripheral side of the objective lens in theapparatus of the aforesaid Item (5) is an area where the numericalaperture for the light flux emitted from the second light source toenter the optical disk is 0.45 based on the maximum numerical aperture(usually, 0.6-0.65) for the light flux emitted from the first lightsource to enter the optical disk.

(f) The area on the peripheral side of the objective lens in theapparatus of the aforesaid Item 46) is an area where the numericalaperture for the light flux emitted from the second light source toenter the optical disk is 0.50 based on the maximum numerical aperture(usually, 0.6-0.65) for the light flux emitted from the first lightsource to enter the optical disk.

A diffractive structure (diffractive section) provided on “the area onthe peripheral side” may be provided either on the side of an objectivelens closer to a light source or on the side of an objective lens closerto an optical information recording medium, or it may further beprovided on both sides thereof, and the diffractive structure isprovided with at least a function to correct temperature characteristicsfor the prescribed light flux passing through the area on the peripheralside.

Incidentally, “the outermost optical surface area” or “the outermostcircumferential optical surface area” means an optical surface area onthe outermost side in the effective diameter, and it is most preferablethat a diffractive structure is provided on that optical surface area.However, it does not affect the invention to provide, without departingfrom the technical spirit and the effect of the invention, a refractionsection having no diffractive structure on a part of the outermostoptical surface area in an effective diameter within a range that a spotdiameter and light intensity both suitable for an optical informationrecording medium (for example, DVD compared with CD) whose necessarynumerical aperture is relatively great are obtained. On the other hand,providing an optical surface area having no influence on recording orreproducing for the optical information recording medium substantiallyon the outermost optical surface area in an effective diameter has noinfluence on the invention. Even when the optical surface area of thiskind exists in the effective diameter, it should be ignored.

Further, “correcting temperature characteristics” means that thefollowing expression is satisfied by change (SA1/∂T) of sphericalaberration for temperature changes, even when a wavelength of a lightsource is changed and a refractive index of the objective lens ischanged both by temperature changes (λ represents a wavelength of alight source).

|δSA1/δT|≦0.002λrms/° C.

In addition, “an average pitch” is assumed to be (a width of an area ofring-shaped diffractive zone in the direction perpendicular to anoptical axis viewed in a section including the optical axis)+(number ofrings in a ring-shaped diffractive zone). Further, “correcting sphericalaberration” is to correct to the level of not more than the diffractionlimit power, and it means that 0.07 λrms and downward (hereat, λrepresents a wavelength of a light source) is satisfied when wave frontaberration is obtained. Further, “m2≈m1” means relationship ofmagnification on the level wherein recording and reproducing for eachoptical information recording medium can be conducted with the samesensor size for both the first optical information recording medium andthe second optical information recording medium. The relationship ofmagnification on the level wherein both recording and reproducing can beconducted with one sensor is more preferable.

With regard to “under spherical aberration or over sphericalaberration”, it is assumed that “under” means an occasion where an imageintersects an optical axis at this side of a paraxial image point, and“over” means an occasion where an image intersects an optical axis atthe far side of a paraxial image point, both in spherical aberrationwhere a position of a paraxial image point is the origin, as shown inFIG. 3.

“Diffractive surface”, “diffractive section”, “diffractive structure” or“ring-shaped diffractive zone” used in the present specification means asection where a relief is provided on the surface of an objective lensto provide a function to converge or diverge a light flux throughdiffraction. With regard to a form of the relief, there is known a formwherein a ring-shaped diffractive zone that is almost in the form ofconcentric circle whose center is an optical axis is formed on thesurface of objective lens as shown in FIG. 4 (b), and a section of thering-shaped diffractive zone on a plane including the optical axis lookslike a serration. The form of the relief includes a form of this kindwhich is especially called “a ring-shaped diffractive zone”.

An objective lens in a narrow sense in the present specification is alens having a light-converging function arranged at the position closestto an optical information recording medium to face it under thecondition that the optical information recording medium is loaded in anoptical pickup device, while an objective lens in a wide sense is a lensgroup capable of being operated by an actuator at least in the directionof its optical axis together with that lens. This lens group in thiscase means at least one or more lenses (for example, two lenses).Therefore, numerical aperture NA of the objective lens on the opticalinformation recording medium side (image side) in the presentspecification means numerical aperture NA of the Lens surface positionedto be closest to the optical information recording medium side on theobjective lens. Further, necessary numerical aperture NA in the presentspecification is a numerical aperture stipulated by specifications ofeach optical information recording medium, or it is a numerical apertureof the objective lens having the diffraction limit power capable ofobtaining a spot diameter necessary for recording or reproducing ofinformation in accordance with a wavelength of a light source used foreach optical information recording medium.

In this specification, the second optical information recording mediummeans CD type optical disks in various types such as, for example, CD=R,CD-RW, CD-Video and CD-ROM, and the first optical information recordingmedium means DVD type optical disks in various types such as DVD-ROM,DVD-RAM, DVD-R, DVD-RW, CD=RW and DVD-Video. Further, thickness t of atransparent base board mentioned in the specification includes t=0. Inaddition, “when using DVD (CD)” means “when conducting recording orreproducing of information for DVD (CD)”.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1( a) to 1(c) are diagrams showing a condition that sphericalaberration is discontinuous.

FIGS. 2( a) to 2(b) are sectional views of an objective lens forillustrating an optical surface area.

FIG. 3 is a diagram showing whether aberration is under or over.

FIGS. 4( a) and 4(b) are diagrams showing a ring-shaped diffractive zoneof a diffractive section. FIG. 4( b) shows a pitch of ring-shapeddiffractive zones.

FIG. 5 is a schematic structure diagram of an optical pickup device.

FIG. 6 is a sectional view showing a schematic structure of an objectivelens of the first embodiment.

FIG. 7 is a schematic structure diagram of an optical pickup device.

FIGS. 8( a) and 8(b) are sectional views showing a schematic structureof an objective lens of the second embodiment. FIG. 8( a) shows acondition that how a light flux is used for the first opticalinformation recording medium (DVD) and FIG. 8( b) shows a condition thathow a light flux is used for the second optical information recordingmedium (CD).

FIG. 9 is a sectional view showing a schematic structure of an objectivelens of the third and fourth embodiments.

FIG. 10 is a sectional view showing a schematic structure of anobjective lens of the fifth embodiment.

FIG. 11 is a spherical aberration diagram for an objective lens inExample 1 where DVD is used.

FIG. 12 is a spherical aberration diagram for an objective lens inExample 1 where CD is used.

FIG. 13 is a spherical aberration diagram for an objective lens inExample 2 where DVD is used.

FIG. 14 is a spherical aberration diagram for an objective lens inExample 2 where CD is used.

FIG. 15 is a sectional view showing a schematic structure of anobjective lens related to a variation example.

FIG. 16 is a spherical aberration diagram for an objective lens inExample 3 where DVD is used.

FIG. 17 is a spherical aberration diagram for an objective lens inExample 3 where CD is used.

FIG. 18 is a spherical is an aberration diagram for an objective lens inExample 4 where DVD is used.

FIG. 19 is a spherical aberration diagram for an objective lens inExample 4 where CD is used.

FIG. 20 is a spherical aberration diagram for an objective lens inExample 5 where DVD is used.

FIG. 21 is a spherical aberration diagram for an objective lens inExample 5 where CD is used.

FIG. 22 is a spherical aberration diagram for an objective lens inExample 6 where DVD is used.

FIG. 23 is a spherical aberration diagram for an objective lens inExample 6 where CD is used.

FIGS. 24( a) and 24(b) are sectional views showing a schematic structureof an objective lens related to another variation example. FIG. 24( a)shows a condition that how a light flux is used for the first opticalinformation recording medium (DVD) and FIG. 24( b) shows a conditionthat how a light flux is used for the second optical informationrecording medium (CD).

FIGS. 25( a) and 25(b) are sectional views showing a schematic structureof an objective lens related to still another variation example. FIG. 25(a) shows a condition that how a light flux is used for the firstoptical information recording medium (DVD) and FIG. 25( b) shows acondition that how a light flux is used for the second opticalinformation recording medium (CD).

FIGS. 26( a) and 26(b) are sectional views showing a schematic structureof an objective lens related to still another variation example. FIG.26( a) shows a condition that how a light flux is used for the firstoptical information recording medium (DVD) and FIG. 26( b) shows acondition that how a light flux is used for the second opticalinformation recording medium (CD).

FIG. 27 is a schematic structure diagram of an optical pickup device.

FIGS. 28( a) and 28(b) are sectional views of primary portions of anobjective lens in the Seventh Embodiment.

FIGS. 29( a) and 29(b) are diagrams showing an example of a design(target characteristics) of spherical aberration related to the SeventhEmbodiment.

FIG. 30 is a sectional view of primary portions of an objective lensrelated to the variation of the Seventh Embodiment.

FIG. 31 is a diagram showing an example wherein a wavelength selectingdiaphragm is provided on an optical pickup device.

FIGS. 32( a) and 32(b) are sectional views of primary portions of anobjective lens related to the Eighth Embodiment.

FIGS. 33( a) and 33(b) are diagrams showing an example of a design(target characteristics) of spherical aberration related to the EighthEmbodiment. FIG. 33( a) show a spherical aberration diagram for DVD andFIG. 33( b) show a spherical aberration diagram for CD.

FIG. 34 is a sectional view of primary portions of an objective lensrelated to the variation of the Eighth Embodiment.

FIG. 35 is a diagram showing an example wherein a coupling lens isprovided on an optical pickup device.

FIG. 36 is a sectional view of primary portions of an objective lensrelated to the Ninth Embodiment.

FIGS. 37( a) and 37(b) are diagrams showing an example of a design(target characteristics) of spherical aberration related to the NinthEmbodiment. FIG. 37( a) show a spherical aberration diagram for DVD andFIG. 37( b) show a spherical aberration diagram for CD.

FIG. 38 is a sectional view of primary portions of an objective lensrelated to the Tenth Embodiment.

FIGS. 39( a) and 39(b) are diagrams showing an example of a design(target characteristics) of spherical aberration related to the TenthEmbodiment. FIG. 39( a) show a spherical aberration diagram for DVD andFIG. 3( b) show a spherical aberration diagram for CD.

FIGS. 40( a) and 40(b) are spherical aberration diagrams of an objectivelens in Example 7. FIG. 40( a) show a spherical aberration diagram forDVD and FIG. 40( b) show a spherical aberration diagram for CD.

FIGS. 41( a) and 41(b) each shows a form of a spot on an informationrecording surface of an optical information recording medium for anobjective lens in the Example 7. FIG. 41( a) is a diagram for CD andFIG. 41( b) is a diagram for DVD.

FIGS. 42( a) and 42(b) are spherical aberration diagrams of an objectivelens in Example 8. FIG. 42( a) show a spherical aberration diagram forDVD and FIG. 42( b) show a spherical aberration diagram for CD.

FIGS. 43( a) and 43(b) each shows a form of a spot on an informationrecording surface of an optical information recording medium for anobjective lens in the Example 8. FIG. 43( a) is a diagram for CD andFIG. 43( b) is a diagram for DVD.

FIGS. 44( a) and 44(b) each is a spherical aberration diagram of anobjective lens in Example 9. FIG. 44( a) show a spherical aberrationdiagram for DVD and FIG. 44( b) show a spherical aberration diagram forCD.

FIGS. 45( a) and 45(b) each shows a form of a spot on an informationrecording surface of an optical information recording medium for anobjective lens in the Example 9. FIG. 45(a) is a diagram for CD and FIG.45( b) is a diagram for DVD.

FIGS. 46( a) and 46(b) each is a spherical aberration diagram of anobjective lens in Example 10. FIG. 46( a) show a spherical aberrationdiagram for DVD and FIG. 46( b) show a spherical aberration diagram forCD.

FIGS. 47( a) and 47(b) each shows a form of a spot on an informationrecording surface of an optical information recording medium for anobjective lens in the Example 10. FIG. 47( a) is a diagram for CD andFIG. 47( b) is a diagram for DVD.

FIGS. 48( a) and 48(b) each is a spherical aberration diagram of anobjective lens in Example 11. FIG. 48( a) show a spherical aberrationdiagram for DVD and FIG. 48( b) show a spherical aberration diagram forCD.

FIGS. 49( a) and 49(b) each shows a form of a spot on an informationrecording surface of an optical information recording medium for anobjective lens in the Example 11. FIG. 49( a) is a diagram for CD andFIG. 49( b) is a diagram for DVD.

FIGS. 50( a) and 50(b) each is a spherical aberration diagram of anobjective lens in Example 12. FIG. 50( a) show a spherical aberrationdiagram for DVD and FIG. 50( b) show a spherical aberration diagram forCD.

FIGS. 51( a) and 51(b) each shows a form of a spot on an informationrecording surface of an optical information recording medium for anobjective lens in the Example 12. FIG. 51( a) is a diagram for CD andFIG. 51( b) is a diagram for DVD.

FIG. 52 is a diagram showing how residual aberration (sphericalaberration) is generated when a thickness of a transparent base board ischanged.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, the invention will further be explained indetail.

First Embodiment

First embodiment will be explained as follows. FIG. 5 is a schematicstructure diagram of an optical pickup device. In optical pickup device100 shown in FIG. 5, a light flux emitted from semiconductor laser 111representing a light source passes through beam splitter 120representing a light merging means, then, stopped down by diaphragm 17to the prescribed numerical aperture, and for a spot on informationrecording surface 220 through diffraction integrated objective lens 160and through transparent base board 210 of high density recording opticaldisk 200 representing an optical information recording medium. Awavelength (standard wavelength) of the semiconductor laser light is 650nm.

A reflected light flux modulated by information bit on informationrecording surface 220 passes through the diffraction-integratedobjective lens 160 again to be changed into a converged light, then,further passes through diaphragm 17 to be reflected on beam splitter 120and passes through cylindrical lens 180 to be subjected to astigmatismand magnification change, and is converged on a light-receiving surfaceof optical detector 300. Incidentally, the numeral 150 in the drawingrepresents an actuator serving as a distance adjusting means for focuscontrol and tracking control. Including an embodiment which will beexplained later, it is preferable that the actuator 150 drives objectivelens 160 in terms of focusing under the state wherein an image formingmagnification is substantially constant.

Incidentally, including an embodiment which will be explained later,when objective lens 160 is driven in terms of tracking in the directionperpendicular to its optical axis by actuator 150, the relative positionof the objective lens 160 to semiconductor laser 111 representing alight source is changed, and in this case, the position where anastigmatism component of wave front aberration of the light fluxemerging out of the objective lens 160 is minimum is a position wherethe optical axis of the objective lens 160 is deviated from the centerof a light flux emitted from the semiconductor laser 111, and therefore,it is possible to expand a range where an astigmatism is smaller thanthe prescribed value. When the distance between the semiconductor laserand an information recording surface of the optical image recordingmedium is made to be greater than 10 mm and to be smaller than 40 mm,optical pickup device 100 can be made compact, which is preferable.

Further, the diaphragm 17 was also established properly to comply withspecifications of the objective lens in the example so that a numericalaperture on the disk 16 side may be a prescribed value. In the presentembodiment, it is also possible to provide a liquid crystal shutter justahead of the diaphragm 17. Incidentally, in the present embodiment andin another embodiment described later, it is conceivable that atemperature sensor that detects a temperature of a semiconductor laserrepresenting a light source is provided, and a temperature of thesemiconductor laser (or an ambient temperature) is adjusted by atemperature adjusting means including a Peltier element by the use ofsignals outputted from the temperature sensor.

FIG. 6 is a sectional schematic view of objective lens 160. On surfaceS1 of the objective lens 160 closer to a light source, there are formedthree optical surface areas A1, A2 and A3. The optical surface area A2between h1 and h2 each representing a height from an optical axis isformed by a refraction section composed of an aspheric surface and eachof the optical surface areas A1 and A3 which are adjacent to the opticalsurface area A2 is formed by a diffractive section.

The optical surface areas A1 that is outside the height h1 determinespower allocation for refraction power and diffraction power of theoutermost diffractive section so that correction of spherical aberrationand correction of temperature characteristics in the course of using DVDmay be the prime object.

Now, when CD is used, over spherical aberration is caused in the designwherein spherical aberration is corrected with a transparent base boardthickness (t₁=0.6 mm) of DVD, because the transparent base boardthickness is greater than the thickness of DVD. As it stands, therefore,recording and reproducing are usually impossible. To realizeinterchangeability, therefore, design of recording and reproducing forCD is conducted for intermediate optical surface area A2. To beconcrete, the design is conducted to correct spherical aberration forthe assumed base board (example, t=0.9 mm) whose thickness is in a rangefrom t₁ to t₂, without making the spherical aberration to be zerothoroughly in CD (t₂=1.2 mm).

On the paraxial optical surface area A3, there is formed a diffractivesection in the same way as in the outermost area A1, and powerallocation for refraction power and diffraction power of the diffractivesection is determined so that correction of spherical aberration andcorrection of temperature characteristics in the course of using DVD maybe the prime object. In this case, generation of spherical aberrationcaused by a difference in transparent base board thickness isproportional to the fourth power of NA, and on the contrary, in the lowNA area, the rate of generation of spherical aberration is less, evenwhen deviated from the designed thickness of the base board. Therefore,by designing properly the paraxial area A3 in which a transparent baseboard thickness for DVD is designed to be t₁ and intermediate opticalsurface area A2, it is possible, even CD is used, to make a light spotformed by optical surface area A3 including an optical axis and byintermediate optical surface area A2 to be not more than the diffractionlimit (0.07 λrms or less: λ represents a wavelength of a light sourcehere), at a certain position on the over side from the paraxial imagepoint.

In the case of using CD, a light flux passing through the outermost areaA1 only turns out to be a flare component, and only a light flux passingthrough the intermediate optical surface area A2 and paraxial opticalsurface area A3 contributes to a CD spot. Though these are not alwaysfree from aberration completely, it is possible to realize an amount ofspherical aberration (about 0.04 λrms) which is especially preferablefor practical use. In the case of using DVD, a light flux passingthrough the intermediate optical surface area A2 only turns out to be aflare component, and a light flux passing through the outermost area A1and paraxial optical surface area A3 is used for forming a spot.Therefore, correction of spherical aberration and correction oftemperature characteristics in the course of using DVD are kept.

Incidentally, the invention is not limited to the aforesaid embodiment.Though the intermediate optical surface area A2 is composed of therefraction section, the same effect is obtained even when theintermediate optical surface area A2 is composed of the diffractivesection having the same spherical aberration. Further, it is naturallypossible to realize even when the refraction section and the diffractivesection exist mixedly on the intermediate optical surface area A2.Further, diffractive sections may be formed on both sides in thedirection of the optical axis. In addition, the paraxial optical surfacearea A3 does not need to be established to be thoroughly free fromaberration in using DVD, and residual aberration of CD may be made lessas shown in the second embodiment described later. In this case,spherical aberration may be caused on the portion close to the opticalaxis.

An optical surface of the objective lens does not need to be composedstrictly of three optical surface areas, and it may be composed of moreoptical surface areas. In that case, it is also possible to arrange sothat at least one optical surface area for correcting a base boardthickness and temperature characteristics in using DVD exists on theoptical surface area outside necessary numerical aperture NA of CD, atleast one optical surface area for forming CD spot exists on at leastone area inside necessary numerical aperture NA of CD, and at least oneoptical surface area for correcting a base board thickness andtemperature characteristics in using DVD exists on the area near anoptical axis.

Second Embodiment

Next, the second embodiment will be explained. This embodiment is onewherein a wavelength of a light source under which DVD is used isdifferent from that under which CD is used, and explanation of portionsin this embodiment which are the same as those in the first embodimentwill be omitted. In the optical pickup device (that is of a type of twolight sources and one detector) related to the present embodiment shownin FIG. 7, there are provided semiconductor laser ill (designedwavelength λ1=650 nm) representing the first light source forreproducing the first optical disk (DVD) and semiconductor laser 112(designed wavelength λ1=780 nm) representing the second light source forreproducing the second optical disk (CD).

First, when reproducing the first optical disk, a beam is emitted fromthe first semiconductor laser 111, and the light flux thus emittedpasses through beam splitter 190 which is a light merging means forlight emitted from the semiconductor laser 111 and for that emitted fromthe semiconductor laser 112, then, passes through beam splitter 120, andis stopped down by diaphragm 17 to be converged by objective lens 160 oninformation recording surface 220 through transparent base board 210 offirst optical disk 200.

Then, the light flux modulated by information bit and reflected on theinformation recording surface 220 passes through the objective lens 160as well as diaphragm 17 again, then, enters the beam splitter 120 to bereflected therein, and is given astigmatism by cylindrical lens 180 toenter optical detector 300, where signals are obtained through readingof information recorded on the first optical disk 200 by the use ofsignals outputted from the optical detector 300.

Further, detection of focusing and detection of tracking are conductedby detecting a change in an amount of light caused by changes in formand position of a spot on the optical detector 300. Based on thisdetection, two-dimension actuator 150 representing a distance adjustingmeans moves objective lens 160 so that a light flux emitted from thefirst semiconductor laser 111 may form images on recording surface 220of the first optical disk 200, and moves objective lens 160 so that alight flux emitted from the first semiconductor laser 111 may formimages on the prescribed track.

When reproducing the second optical disk, a beam is emitted from thesecond semiconductor laser 112, and the light flux thus emitted isreflected on beam splitter 190 which is a light merging means, and isconverged on information recording surface 220 through beam splitter120, diaphragm 17 and objective lens 160 in the same way as in the lightflux emitted from the first semiconductor 111, and through transparentbase board 210 of the second optical disk 200.

Then, the light flux modulated by information bit and reflected oninformation recording surface 220 enters the optical detector 300through the objective lens 160, diaphragm 17, beam splitter 120 andcylindrical lens 180 again, and signals are obtained through reading ofinformation recorded on the second optical disk 200 by the use ofsignals outputted from the optical detector 300.

In the same way as in the first optical disk, detection of focusing anddetection of tracking are conducted by detecting a change in an amountof light caused by changes in a form and a position of the spot on theoptical detector 300, and two-dimension actuator 150 moves objectivelens 160 for focusing and tracking.

FIG. 8 shows a schematic sectional view of an objective lens. On surfaceS1 of the objective lens 160 closer to a light source, there are formedthree optical surface areas A1, A2 and A3. Each optical surface area iscomposed of a diffractive section, and outermost optical surface area A1and optical surface area A3 near an optical axis are diffractionsurfaces under the same design concept, while, intermediate opticalsurface area A2 between h1 and h2 each representing a height from anoptical axis is a diffractive section designed from a viewpoint that isdifferent from that for diffractive sections on both sides of theintermediate optical surface area A2.

The outermost optical surface area A1 and optical surface area A3 nearan optical axis conduct correction of a base board thickness andcorrection of temperature characteristics in the course of using DVD.When using CD, in this case, under spherical aberration is generated onthe light flux passing through the aforesaid diffractive section asspherical aberration for the color corresponding to the wavelength ofthe light source that is longer compared with that for DVD. In thiscase, to make it possible to conduct reproducing and recording for CD,the optical design of intermediate optical surface area A2 is made sothat spherical aberration which is different from that for thediffractive sections on both sides may be given to the intermediateoptical surface area A2. Even in the present embodiment, sphericalaberration is not made to be zero perfectly in CD (t₂=1.2 mm), but abase board (for example, t=0.9 mm) having a certain thickness between t₁and t₂ is assumed, and spherical aberration is corrected for that baseboard, in the design. Though the corresponding portion has underspherical aberration when using DVD, it turns out to be flare lightwhich is far from the main spot.

On the other hand, when using CD, a light flux passing through theoutermost optical surface area A1 only turns out to be flare component,and those contributing to CD spot are only intermediate optical surfacearea A2 and optical surface area A3 near an optical axis (see FIG. 8(b)). Though these are not free from aberration completely, an amount ofspherical aberration capable of being used practically (about 0.04 λrms)can be realized. When using DVD, a light flux passing throughintermediate optical surface area A2 is a flare component (see FIG. 8(a)), and outermost optical surface area A1 and optical surface area A3near an optical axis are used for forming the spot. Therefore,interchangeability with CD can be realized under the condition wherecorrection of spherical aberration and correction of temperaturecharacteristics are kept in the course of using DVD.

Incidentally, the invention is not limited to the aforesaid embodiment.Though the intermediate optical surface area A2 is composed of therefraction section, the same effect is obtained even when theintermediate optical surface area A2 is composed of the diffractivesection having the same spherical aberration. Further, it is naturallypossible to realize even when the refraction section and the diffractivesection exist mixedly on the intermediate optical surface area A2.Further, diffractive sections may be formed on both sides in thedirection of the optical axis. In addition, the paraxial optical surfacearea A3 does not need to be established to be thoroughly free fromaberration in using DVD, and residual aberration of CD may be made less.In this case, spherical aberration may be caused on the portion close tothe optical axis.

An optical surface of the objective lens does not need to be composedstrictly of three optical surface areas, and it may be composed of moreoptical surface areas. In that case, it is also possible to arrange sothat at least one optical surface area for correcting a base boardthickness and temperature characteristics in using DVD exists on theoptical surface area outside necessary numerical aperture NA of CD, atleast one optical surface area for forming CD spot exists on at leastone area inside necessary numerical aperture NA of CD, and at least oneoptical surface area for correcting a base board thickness andtemperature characteristics in using DVD exists on the area near anoptical axis.

Third Embodiment

Next, the third embodiment will be explained. This embodiment is onewherein a wavelength of a light source under which DVD is used is thesame as that under which CD is used, and explanation of portions in thisembodiment which are the same as those in the aforesaid embodiment willbe omitted. An optical pickup device is the same as one shown in FIG. 5in terms of structure. A schematic structure diagram of an objectivelens is shown in FIG. 9.

On surface S1 of objective lens 160 closer to a light source, there areformed three optical surface areas A1, A2 and A3 each being designedoptically based on a different concept. However, from the viewpoint ofusing a light flux, a light flux passing through the outermost opticalsurface area A1 and the innermost optical surface area A3 is used toform an optical spot on a recording surface in the case of using DVD,and a light flux passing through the intermediate optical surface areaA2 and the innermost optical surface area A3 is used to form an opticalspot in the case of using CD, in the same way as in the embodimentexplained already.

A diffraction surface of optical surface area A1 outside h1 representinga height from optical axis X is designed for correction of a base boardthickness and temperature characteristics in the case of using DVD, inthe same way as in the first embodiment, and when using CD, over flarelight is generated. Intermediate optical surface area A2 is designed tocorrect spherical aberration for the assumed base board having a certainthickness between t₁ and t₂ (for example, t=0.9 mm) for a purpose ofinterchangeability with CD, and it is used for forming a spot in thecase of using CD, and an under flare light is generated when DVD is usedon the innermost optical surface area A3, the refraction surface isdesigned for correcting a base board thickness of DVD basically, and aform of spherical aberration on the portion near an optical axis isdevised for lessening residual aberration in the case of using CD. Thisarea is also used for forming main spot light for DVD and CD, which hasbeen described already.

Incidentally, the invention is not limited to the aforesaid embodiment.Though the intermediate optical surface area A2 is composed of therefraction section, the same effect is obtained even when theintermediate optical surface area A2 is composed of the diffractivesection having the same spherical aberration. Further, it is naturallypossible to realize even when the refraction section and the diffractivesection exist mixedly on the intermediate optical surface area A2.Further, diffractive sections may be formed on both sides in thedirection of the optical axis. In addition, the paraxial optical surfacearea A3 does not need to be established to be thoroughly free fromaberration in using DVD, and residual aberration of CD may be made less.In this case, spherical aberration may be caused on the portion close tothe optical axis.

An optical surface of the objective lens does not need to be composedstrictly of three optical surface areas, and it may be composed of moreoptical surface areas. In that case, it is also possible to arrange sothat at least one optical surface area for correcting a base boardthickness and temperature characteristics in using DVD exists on theoptical surface area outside necessary numerical aperture NA of CD, atleast one optical surface area for forming CD spot exists on at leastone area inside necessary numerical aperture NA of CD, and at least oneoptical surface area for correcting a base board thickness andtemperature characteristics in using DVD exists on the area near anoptical axis.

Fourth Embodiment

Next, the fourth embodiment will be explained. This embodiment is onewherein a wavelength of a light source under which DVD is used isdifferent from that under which CD is used, and an optical pickup deviceis the same as one shown in FIG. 7 in terms of structure. A schematicsectional view of an objective lens is the same as one shown in FIG. 9.

On a surface of an objective lens closer to a light source, there areformed three optical surface areas A1, A2 and A3 each being designedoptically based on a different concept. However, from the viewpoint ofusing a light flux, a light flux passing through the out side and theinside is used to form an spot light on a recording surface in the caseof using DVD, and a light flux passing through the intermediate portionand the inside is used to form a spot light in the case of using CD, inthe same way as in the embodiment explained already.

A diffraction surface of optical surface area A1 outside h1 representinga height from optical axis X is designed for correction of a base boardthickness and temperature characteristics in the case of using DVD, inthe same way as in the first embodiment, and when using CD, under flarelight is generated. Intermediate optical surface area A2 is designed tocorrect spherical aberration for the assumed base board having a certainthickness between t₁ and t₂ (for example, t=0.9 mm) for a purpose ofinterchangeability with CD, and it is used for forming a spot in thecase of using CD, and an over flare light is generated when DVD is used.On the innermost optical surface area A3, the refraction surface isdesigned for correcting a base board thickness of DVD basically, and aform of spherical aberration on the portion near an optical axis isdevised for lessening residual aberration in the case of using CD.Spherical aberration of this area generated when CD is used is under onewhich is opposite to that in the third embodiment. This area is alsoused for forming main spot light for DVD and CD, which has beendescribed already.

Incidentally, the invention is not limited to the aforesaid embodiment.Though the intermediate optical surface area A2 is composed of therefraction section, the same effect is obtained even when theintermediate optical surface area A2 is composed of the diffractivesection having the same spherical aberration. Further, it is naturallypossible to realize even when the refraction section and the diffractivesection exist mixedly on the intermediate optical surface area A2.Further, diffractive sections may be formed on both sides in thedirection of the optical axis. In addition, the paraxial optical surfacearea A3 does not need to be established to be thoroughly free fromaberration in using DVD, and residual aberration of CD may be made less.In this case, spherical aberration may be caused on the portion close tothe optical axis.

An optical surface of the objective lens does not need to be composedstrictly of three optical surface areas, and it may be composed of moreoptical surface areas. In that case, it is also possible to arrange sothat at least one optical surface area for correcting a base boardthickness and temperature characteristics in using DVD exists on theoptical surface area outside necessary numerical aperture NA of CD, atleast one optical surface area for forming CD spot exists on at leastone area inside necessary numerical aperture NA of CD, and at least oneoptical surface area for correcting a base board thickness andtemperature characteristics in using DVD exists on the area near anoptical axis.

Fifth Embodiment

Next, the fifth embodiment will be explained. This embodiment is onewherein a wavelength of a light source under which DVD is used is thesame as that under which CD is used, and an optical pickup device is thesame as one shown in FIG. 5 in terms of structure. A schematic structurediagram of an objective lens is shown in FIG. 10.

On surface S1 of objective lens 160 closer to a light source, there areformed two optical surface areas A1 and A2 each being designed opticallybased on a different concept. From the viewpoint of using a light flux,a light flux passing through the outside and the inside is used to forma spot light on a recording surface in the case of using DVD, and alight flux passing through the inside is used to form a spot light on arecording surface in the case of using CD.

A diffraction surface of optical surface area A1 outside h1 representinga height from optical axis X is designed for correction of a base boardthickness and temperature characteristics in the case of using DVD, inthe same way as in the first embodiment, and when using CD, over flarelight is generated. Inside optical surface area A2 is designed tocorrect spherical aberration for the assumed base board having a certainthickness between t₁ and t₂ (for example, t=0.9 mm) for a purpose ofinterchangeability with CD, and it is used for forming a spot in thecase of using CD, and it is used to contribute to forming a spot lightwhen DVD is used. Further, a form of spherical aberration on the portionnear an optical axis is devised for lessening residual aberration in thecase of using CD. Spherical aberration generated on this area when CD isused is under spherical aberration which is opposite to that in thethird embodiment. This area is also used for forming main spot light forDVD and CD, which has been described already. Incidentally, theinvention is not limited to the aforesaid embodiment. Though the insideoptical surface area A2 is composed of the refraction section, the sameeffect is obtained even when the inside optical surface area A2 iscomposed of the diffractive section having the same sphericalaberration. Further, it is naturally possible to realize even when thediffractive section and the refraction section exist mixedly on theintermediate optical surface area A2. Further, diffractive sections maybe formed on both sides in the direction of the optical axis.

Sixth Embodiment

Next, the sixth embodiment will be explained. This embodiment is onewherein a wavelength of a light source under which DVD is used isdifferent from that under which CD is used, and an optical pickup deviceis the same as one shown in FIG. 7 in terms of structure. A schematicsectional view of an objective lens is shown in FIG. 15.

On surface S1 of objective lens 160 closer to a light source, there areformed two optical surface areas A1 and A2 each being designed opticallybased on a different concept. From the viewpoint of using a light flux,a light flux passing through the outside and the inside is used to forma spot light on a recording surface in the case of using DVD, and alight flux passing through the inside is used to form a spot light on arecording surface in the case of using CD.

A diffraction surface of optical surface area A1 outside h1 representinga height from optical axis X is designed for correction of a base boardthickness and temperature characteristics in the case of using DVD, inthe same way as in the first embodiment, and when using CD, over flarelight is generated. Intermediate optical surface area A2 is designed tocorrect spherical aberration for the assumed base board having a certainthickness between t₁ and t₂ (for example, t=0.9 mm) while utilizingspherical aberration for the color corresponding to the longer length interms of a length of a light source compared with DVD, for a purpose ofinterchangeability with CD, and it is used for forming a spot in thecase of using CD, and it is used to contribute to forming a spot lightwhen DVD is used.

Therefore, when using CD, a light flux passing through the outsideoptical surface area A1 only turns out to be flare component, and whatis contributing to forming of a spot light for CD is a light fluxpassing through the inside optical surface area A2, and when using DVD,a light flux passing through the outside optical surface area A1 and alight flux passing through the inside optical surface area A2 are usedfor forming a spot light. Therefore, interchangeability with CD can berealized under the condition where correction of spherical aberrationand correction of temperature characteristics are kept in the course ofusing DVD.

Further, in many actual optical pickup devices, a distance between anemission point and each disk surface is constant, and there is a highpossibility that an actual image forming magnification for DVD isdifferent from that for CD. However, the distance between an emissionpoint and a lens surface is made to be the same for DVD and CD in thefollowing examples, because that strictness does not matter in substanceof the invention.

Incidentally, the invention is not limited to the present embodiment.Though a diffractive section is used to constitute the inside opticalsurface area A2, the effect is the same even when a refraction sectionhaving the same spherical aberration is used. Further, even when thediffractive section and the refraction section exit mixedly on theinside optical surface area A2, it is naturally possible to realize. Inaddition, the diffractive section may further be formed on both sides inthe direction of an optical axis.

Examples of the objective lens which is favorably used in the opticalpickup device in the embodiment described above will be explained asfollows.

In general, a pitch of a ring-shaped diffractive zone on the diffractionsurface is defined by using a phase difference function or an opticalpath difference function. To be concrete, phase difference function isexpressed by the following “Numeral 1” in a unit of radian, and opticalpath difference function ΦB is expressed by the following “Numeral 2” ina unit of mm.

$\begin{matrix}{\Phi_{b} = {\sum\limits_{i = 1}^{\infty}{b_{2i}h^{2i}}}} & \left( {{Numeral}\mspace{20mu} 1} \right) \\{\Phi_{B} = {\sum\limits_{i = 1}^{\infty}{B_{2i}h^{2i}}}} & \left( {{Numeral}\mspace{20mu} 2} \right)\end{matrix}$

These two expression methods are different each other in terms of aunit, but they are the same in terms of expressing a pitch of aring-shaped diffractive zone. Namely, if phase difference functioncoefficient b is multiplied by λ/2π for main wavelength λ (unit mm), itis possible to convert into optical path difference function coefficientB, while, if optical path difference function coefficient B is dividedby λ/2π on the contrary, it is possible to convert into phase differencefunction coefficient b.

Based on the definition stated above, it is possible to make a lens tohave power, by making the secondary coefficient of the phase differencefunction or of the optical path difference function to be the valueother than zero. Further, it is possible to control spherical aberrationby making the coefficient of the phase difference function or of theoptical path difference function other than the secondary coefficient,for example, quaternary coefficient, 6-th order coefficient, 8-th ordercoefficient and 10-th order coefficient. Controlling in this case meansthat spherical aberration is corrected on the whole by giving oppositespherical aberration to the diffractive section for spherical aberrationof the refraction section or that the total spherical aberration is madeto be a desired flare a amount by manipulating spherical aberration ofthe diffractive section.

In addition, the diffraction surface mentioned above is formed on thesurface on at least one side, and that surface has thereon an asphericalform expressed by the following expression “Numeral 3”.

$\begin{matrix}{Z = {\frac{h^{2}/R_{0}}{1 + \sqrt{1 - {\left( {1 + \kappa} \right)\left( {h/R_{0}} \right)^{2}}}} + {\sum\limits_{i = 1}^{\infty}{A_{i}h^{Pi}}}}} & \left( {{Numeral}\mspace{20mu} 3} \right)\end{matrix}$

In the expression, Z represents an axis in the direction of an opticalaxis, h represents an axis in the direction perpendicular to an opticalaxis (height from an optical axis: advancing direction of light ispositive), R0 represents a paraxial radius of curvature, κ representsthe constant of the cone, A represents the aspherical coefficient and Prepresents the number of power of the aspheric surface.

Incidentally, from now on (including lens data of the table), the powermultiplier of 10 (for example, 2.5×10⁻³) is shown by the use of use E(for example, 2.5×E-3).

Example 1

With regard to the example of the objective lens which can be used forthe Embodiment 1 mentioned above, data of the objective lens are shownin Table 1. FIG. 11 is a spherical aberration diagram for DVD and FIG.12 is that for CD. Necessary numerical aperature NA of DVD is 0.60 andthat of CD is 0.45.

TABLE 1 Example 1 f₁ = 3.05 mm, f2 = 3.05 mm, m1 = −1/6.01, m2 = −1/6.01NAH = 1.373 mm, NAL = 1.22 mm Pout = 0.00367 mm, Pin = 0.04368 mm n = 1δSA1/δT = 0.0001 λrms/° C. δSA/δU = 0.063 λrms/mm DVD CD ith ni nisurface ri di (650 nm) (650 nm) di (650 nm) (650 nm) 0 20.006 1.0 20.0061.0 Emission point 1 ∞ 0.0 1.0 0.0 1.0 Aperture φ4.452 mm 2 2.11184 1.721.54094 1.72 1.54094  2′ 2.11184 1.72 1.54094 1.72 1.54094  2″ 2.111841.72 1.54094 1.72 1.54094 3 −5.3457 2.20 1.0 1.83 1.0 4 ∞ 0.6 1.5778661.2 1.577866 5 ∞ Aspherical data 2nd surface (0 < h < 1.22 mm: Opticalsurface area including optical axis) Aspherical coefficient κ −1.6695 ×E−0 A1 +1.0619 × E−2 P1 4.0 A2 −1.6783 × E−3 P2 6.0 A3 +1.2711 × E−4 P38.0 A4 +1.9174 × E−8 P4 10.0 Optical path difference function(Coefficient of optical path difference function: Standard wavelength650 nm) B2 −3.8401 × E−3 B4 −1.2957 × E−4 B6 −2.8158 × E−5 B8 +9.8536 ×E−6 B10 −1.9454 × E−7 2′nd surface (1.22 mm < h < 1.373 mm: Intermediateoptical surface area) Aspherical coefficient κ −1.6536 × E−0 A1 +1.0637× E−2 P1 4.0 A2 −1.6905 × E−3 P2 6.0 A3 +1.2505 × E−4 P3 8.0 A4 −1.7615× E−7 P4 10.0 Optical path difference function (Coefficient of opticalpath difference function: Standard wavelength 650 nm) B2 −3.8920 × E−3B4 −1.3036 × E−4 B6 −2.4328 × E−5 B8 +1.1263 × E−5 B10 −1.3503 × E−62″nd surface (1.373 mm < h: Outside optical surface area) Asphericalcoefficient κ −1.6695 × E−0 A1 +1.0619 × E−2 P1 4.0 A2 −1.6783 × E−3 P26.0 A3 +1.2711 × E−4 P3 8.0 A4 +1.9174 × E−8 P4 10.0 Optical pathdifference function (Coefficient of optical path difference function:Standard wavelength 650 nm) B2 −3.8401 × E−3 B4 −1.2957 × E−4 B6 −2.8158× E−5 B8 +9.8536 × E−6 B10 −1.9454 × E−7 3rd surface Asphericalcoefficient κ −3.1740 × E+1 A1 +4.1021 × E−3 P1 4.0 A2 −6.9699 × E−4 P26.0 A3 +6.7716 × E−5 P3 8.0 A4 −6.4184 × E−6 P4 10.0 A5 +1.8509 × E−7 P512.0

Example 2

With regard to the example of the objective lens which can be used forthe Embodiment 2 mentioned above, data of the objective lens are shownin Table 2. FIG. 13 is spherical aberration diagram for DVD and FIG. 14is that for CD. Necessary numerical aperature NA of DVD is 0.60 and thatof CD is 0.45.

TABLE 2 Example 2 f₁ = 3.05 mm, f2 = 3.06 mm, m1 = −1/6.01, m2 = −1/5.97NAH = 1.370 mm, NAL = 0.81 mm Pout = 0.00369 mm, Pin = 0.1600 mm n = 1δSA1/δT = 0.0001 λrms/° C. δSA/δU = 0.063 λrms/mm DVD CD ith ni nisurface ri di (650 nm) (650 nm) di (780 nm) (780 nm) 0 20.006 1.0 20.0061.0 Emission point 1 ∞ 0.0 1.0 0.0 1.0 Aperture φ4.452 mm 2 2.11184 1.721.54094 1.72 1.53729  2′ 2.11184 1.72 1.54094 1.72 1.53729  2″ 2.111841.72 1.54094 1.72 1.53729 3 −5.3457 2.20 1.0 1.83 1.0 4 ∞ 0.6 1.5778661.2 1.570839 5 ∞ Aspherical data 2nd surface (0 < h < 0.81 mm: Opticalsurface area including optical axis) Aspherical coefficient κ −1.6695 ×E−0 A1 +1.0619 × E−2 P1 4.0 A2 −1.6783 × E−3 P2 6.0 A3 +1.2711 × E−4 P38.0 A4 +1.9174 × E−8 P4 10.0 Optical path difference function(Coefficient of optical path difference function: Standard wavelength650 nm) B2 −3.8401 × E−3 B4 −1.2957 × E−4 B6 −2.8158 × E−5 B8 +9.8536 ×E−6 B10 −1.9454 × E−7 2′nd surface (0.81 mm < h < 1.370 mm: Intermediateoptical surface area) Aspherical coefficient κ −1.5361 × E−0 A1 +1.2030× E−2 P1 4.0 A2 −7.7324 × E−4 P2 6.0 A3 +4.5188 × E−4 P3 8.0 A4 −1.3696× E−4 P4 10.0 Optical path difference function (Coefficient of opticalpath difference function: Standard wavelength 780 nm) B2 −2.5830 × E−3B4 +3.8438 × E−4 B6 +2.0764 × E−5 B8 −1.9229 × E−5 B10 −8.1530 × E−62″nd surface (1.370 mm < h: Outside optical surface area) Asphericalcoefficient κ −1.6695 × E−0 A1 +1.0619 × E−2 P1 4.0 A2 −1.6783 × E−3 P26.0 A3 +1.2711 × E−4 P3 8.0 A4 +1.9174 × E−8 P4 10.0 Optical pathdifference function (Coefficient of optical path difference function:Standard wavelength 650 nm) B2 −3.8401 × E−3 B4 −1.2957 × E−4 B6 −2.8158× E−5 B8 +9.8536 × E−6 B10 −1.9454 × E−7 3rd surface Asphericalcoefficient κ −3.1740 × E+1 A1 +4.1021 × E−3 P1 4.0 A2 −6.9699 × E−4 P26.0 A3 +6.7716 × E−5 P3 8.0 A4 −6.4184 × E−6 P4 10.0 A5 +1.8509 × E−7 P512.0

Example 3

With regard to the example of the objective lens which can be used forthe Embodiment 6 mentioned above, data of the objective lens are shownin Table 2. FIG. 16 is a spherical aberration diagram for DVD and FIG.17 is that for CD. Necessary numerical aperture NA of DVD is 0.60 andthat of CD is 0.45.

TABLE 3 f₁ = 3.20 mm, f2 = 3.21 mm, m1 = −1/6.8, m2 = −1/6.8 NAH =1.66681 mm Pout = 0.0217 mm, Pin = 0.111 mm n = 1 δSA2/δT = 0.00077λrms/° C. δSA1/δU = 0.066 λrms/mm ith ni ni surface ri di (655 nm) (655nm) di (785 nm) (785 nm) 0 2.43289 24.699 1 ∞ 0.0 1.0 0.0 1.0 Apertureφ4.3108 mm 2 2.219924 2.6 1.54094 2.6 1.53716  2′ 2.321811 2.59381.54094 2.5938 1.53716 3 −4.6282 1.97666 1.0 1.60656 1.0 4 ∞ 0.6 1.577521.2 1.57063 5 ∞ Aspherical data 2nd surface (0 < h < 1.66681 mm: Opticalsurface area including optical axis) Aspherical coefficient κ  −2.0664 ×E−0 A1  +1.4172 × E−2 P1 4.0 A2  +1.8597 × E−4 P2 6.0 A3  −7.6246 × E−4P3 8.0 A4  +2.9680 × E−4 P4 10.0 A5  −5.9552 × E−5 P5 12.0 A6  +5.2766 ×E−6 P6 14.0 Optical path difference function (Coefficient of opticalpath difference function: Standard wavelength 720 nm) B4  −1.9684 × E−3B6  +5.8778 × E−4 B8  −1.7198 × E−4 B10  +1.8183 × E−5 2′nd surface(1.66681 mm < h: Outside optical surface area) Aspherical coefficient κ −5.2521 × E−1 A1  +7.2310 × E−3 P1 4.0 A2  −5.3542 × E−3 P2 6.0 A3 +1.6587 × E−3 P3 8.0 A4  −2.9617 × E−4 P4 10.0 A5  +3.0030 × E−5 P512.0 A6  −1.6742 × E−6 P6 14.0 Optical path difference function(Coefficient of optical path difference function: Standard wavelength655 nm) B2  +2.7391 × E−3 B4  −4.3035 × E−3 B6  +1.1732 × E−3 B8 −1.6358 × E−4 B10  +7.6874 × E−6 3rd surface Aspherical coefficient κ−2.14215 × E−0 A1 +3.14404 × E−2 P1 4.0 A2 −1.58639 × E−2 P2 6.0 A3+6.63865 × E−3 P3 8.0 A4 −1.73208 × E−3 P4 10.0 A5 +2.34860 × E−4 P512.0 A6 −1.30087 × E−5 P6 14.0

Example 4

With regard to another example of the objective lens which can be usedfor the Embodiment 6 mentioned above, data of the objective lens areshown in Table 2. FIG. 18 is a spherical aberration diagram for DVD andFIG. 19 is that for CD. Necessary numerical aperture NA of DVD is 0.60and that of CD is 0.45.

TABLE 4 f₁ = 3.20 mm, f2 = 3.21 mm, m1 = −1/6.8, m2 = −1/6.8 NAH =1.66681 mm Pout = 0.0190 mm, Pin = 0.111 mm n = 1 δSA2/δT = 0.00070λrms/° C. δSA1/δU = 0.054 λrms/mm ith ni ni surface ri di (655 nm) (655nm) di (785 nm) (785 nm) 0 24.3312 24.7024 1 ∞ 0.0 1.0 0.0 1.0 Apertureφ4.3108 mm 2 2.21708 2.6 1.54094 2.6 1.53716  2′ 2.315273 2.5938 1.540942.5938 1.53716 3 −4.6451 1.9744 1.0 1.6032 1.0 4 ∞ 0.6 1.57752 1.21.57063 5 ∞ Aspherical data 2nd surface (0 < h < 1.66681 mm: Opticalsurface area including optical axis) Aspherical coefficient κ  −1.9916 ×E−0 A1  +1.2271 × E−2 P1 4.0 A2  +2.6623 × E−4 P2 6.0 A3  −4.8051 × E−4P3 8.0 A4  +9.4489 × E−5 P4 10.0 A5  −2.6250 × E−6 P5 12.0 A6  −1.0534 ×E−6 P6 14.0 Optical path difference function (Coefficient of opticalpath difference function: Standard wavelength 720 nm) B4  −2.3605 × E−3B6  +8.0849 × E−4 B8  −2.1222 × E−4 B10  +1.7503 × E−5 2′nd surface(1.66681 mm < h: Outside optical surface area) Aspherical coefficient κ −5.5582 × E−1 A1  +6.7989 × E−3 P1 4.0 A2  −5.4908 × E−3 P2 6.0 A3 +1.6536 × E−3 P3 8.0 A4  −2.9300 × E−4 P4 10.0 A5  +3.0799 × E−5 P512.0 A6  −1.7778 × E−6 P6 14.0 Optical path difference function(Coefficient of optical path difference function: Standard wavelength655 nm) B2  +2.8609 × E−3 B4  −4.3411 × E−3 B6  +1.1344 × E−3 B8 −1.6710 × E−4 B10  +9.1424 × E−6 3rd surface Aspherical coefficient κ−6.70263 × E−1 A1 +2.98350 × E−2 P1 4.0 A2 −1.51427 × E−2 P2 6.0 A3+6.64091 × E−3 P3 8.0 A4 −1.74128 × E−3 P4 10.0 A5 +2.32281 × E−4 P512.0 A6 −1.25448 × E−5 P6 14.0

Example 5

With regard to another example of the objective lens which can be usedfor the Embodiment 6 mentioned above, data of the objective lens areshown in Table 2. FIG. 20 is a spherical aberration diagram for DVD andFIG. 21 is that for CD. Necessary numerical aperture NA of DVD is 0.60and that of CD is 0.45.

TABLE 5 f₁ = 3.20 mm, f2 = 3.21 mm, m1 = −1/6.8, m2 = −1/6.8 NAH =1.66681 mm Pout = 0.0144 mm, Pin = 0.0556 mm n = 1 δSA2/δT = 0.00102λrms/° C. δSA1/δU = 0.057 λrms/mm ith ni ni surface ri di (655 nm) (655nm) di (785 nm) (785 nm) 0 24.3403 24.7307 1 ∞ 0.0 1.0 0.0 1.0 Aperture4.3108 mm 2 2.28859 2.6 1.54094 2.6 1.53716  2′ 2.43366 2.5928 1.540942.5928 1.53716 3 −4.7132 1.9653 1.0 1.5749 1.0 4 ∞ 0.6 1.57752 1.21.57063 5 ∞ Aspherical data 2nd surface (0 < h < 1.66681 mm: Opticalsurface area including optical axis) Aspherical coefficient κ  −1.0061 ×E−0 A1  +4.2439 × E−3 P1 4.0 A2  −1.4759 × E−3 P2 6.0 A3  +9.3408 × E−4P3 8.0 A4  −5.1099 × E−4 P4 10.0 A5  +1.5021 × E−4 P5 12.0 A6  −1.5815 ×E−5 P6 14.0 Optical path difference function (Coefficient of opticalpath difference function: Standard wavelength 720 nm) B2  −4.8645 × E−3B4  −7.2782 × E−4 B6  −1.8032 × E−4 B8  −4.9114 × E−6 B10  +1.3132 × E−52′nd surface (1.66681 mm < h: Outside optical surface area) Asphericalcoefficient κ  −7.9917 × E−1 A1  +1.2236 × E−2 P1 4.0 A2  −5.6577 × E−3P2 6.0 A3  +1.6609 × E−3 P3 8.0 A4  −2.9009 × E−4 P4 10.0 A5  +2.9096 ×E−5 P5 12.0 A6  −1.5424 × E−6 P6 14.0 Optical path difference function(Coefficient of optical path difference function: Standard wavelength655 nm) B2  −2.8166 × E−3 B4  −3.1771 × E−3 B6  +1.0641 × E−3 B8 −1.9508 × E−4 B10  +1.2278 × E−5 3rd surface Aspherical coefficient κ−5.47493 × E−1 A1 +2.95069 × E−2 P1 4.0 A2 −1.46461 × E−2 P2 6.0 A3+6.39635 × E−3 P3 8.0 A4 −1.71136 × E−3 P4 10.0 A5 +2.35330 × E−4 P512.0 A6 −1.31514 × E−5 P6 14.0

Example 6

With regard to still another example of the objective lens which can beused for the Embodiment 6 mentioned above, data of the objective lensare shown in Table 2. FIG. 22 is a spherical aberration diagram for DVDand FIG. 23 is that for CD. Necessary numerical aperture NA of DVD is0.60 and that of CD is 0.45.

TABLE 6 f₁ = 3.20 mm, f2 = 3.21 mm, m1 = −1/6.8, m2 = −1/6.8 NAH =1.66681 mm Pout = 0.0135 mm, Pin = 0.0450 mm n = 1 δSA2/δT = 0.00097λrms/° C. δSA1/δU = 0.057 λrms/mm ith ni ni surface ri di (655 nm) (655nm) di (785 nm) (785 nm) 0 24.3320 24.7315 1 ∞ 0.0 1.0 0.0 1.0 Aperture4.3108 mm 2 2.32575 2.6 1.54094 2.6 1.53716  2′ 2.45552 2.5963 1.540942.5963 1.53716 3 −4.6504 1.9653 1.0 1.5749 1.0 4 ∞ 0.6 1.57752 1.21.57063 5 ∞ Aspherical data 2nd surface (0 < h < 1.66681 mm: Opticalsurface area including optical axis) Aspherical coefficient κ  −1.1171 ×E−0 A1  +3.1061 × E−3 P1 4.0 A2  +1.6363 × E−3 P2 6.0 A3  −1.1145 × E−3P3 8.0 A4  +3.1702 × E−4 P4 10.0 A5  −4.9061 × E−5 P5 12.0 A6  +5.3895 ×E−6 P6 14.0 Optical path difference function (Coefficient of opticalpath difference function: Standard wavelength 720 nm) B2  −6.3187 × E−3B4  −1.7269 × E−3 B6  +8.2815 × E−4 B8  −4.0856 × E−4 B10  +6.8845 × E−52′nd surface (1.66681 mm < h: Outside optical surface area) Asphericalcoefficient κ  −8.2400 × E−1 A1  +1.1865 × E−2 P1 4.0 A2  −5.4663 × E−3P2 6.0 A3  +1.6917 × E−3 P3 8.0 A4  −2.9856 × E−4 P4 10.0 A5  +2.6842 ×E−5 P5 12.0 A6  −1.1008 × E−6 P6 14.0 Optical path difference function(Coefficient of optical path difference function: Standard wavelength655 nm) B2  −5.3662 × E−3 B4  −2.7368 × E−3 B6  +1.0893 × E−3 B8 −2.3018 × E−4 B10  +1.6566 × E−5 3rd surface Aspherical coefficient κ−1.22207 × E−0 A1 +3.03718 × E−2 P1 4.0 A2 −1.45690 × E−2 P2 6.0 A3+6.19508 × E−3 P3 8.0 A4 −1.71672 × E−3 P4 10.0 A5 +2.51638 × E−4 P512.0 A6 −1.50897 × E−5 P6 14.0

Table 7 shows refractive indexes of the objective lens and of thetransparent base board of the optical information recording medium foreach wavelength, and temperature characteristics data of thesemiconductor laser (light source).

TABLE 7 Refractive index of Refractive index of transparent baseobjective lens board 644 nm 1.5412 1.5783 650 nm 1.5409 1.5779 656 nm1.5407 1.5775 780 nm 1.5373 1.5708 δn/δT/(/° C.) −1.2 × 10⁻⁵ −1.4 × 10⁵Temperature characteristics δλ/δT/ = +0.2 nm/° C.) of wavelength emittedfrom light source

In the examples stated above, Example 1 exemplifies the objective lenswherein outermost optical surface area A1 is composed of a diffractivesection, intermediate optical surface area A2 is composed of arefraction section and near-optical-axis optical surface area A3 iscomposed of a diffractive section, as shown in FIG. 6, and Example 2exemplifies the objective lens wherein outermost optical surface area A1is composed of a diffractive section as shown in FIG. 8. However, it isalso possible to employ the constitution wherein outermost opticalsurface area A1 is composed of a diffractive section, intermediateoptical surface area A2 is composed of a mixture of a diffractivesection and a refraction section and near-optical-axis optical surfacearea A3 is composed of a diffractive section, as shown in FIG. 24. It isfurther possible to employ the constitution wherein outermost opticalsurface area A1 is composed of a diffractive section, intermediateoptical surface area A2 is composed of a diffractive section andnear-optical-axis optical surface area A3 is composed of a refractionsection, as shown in FIG. 9, the constitution wherein outermost opticalsurface area A1 is composed of a diffractive section, intermediateoptical surface area A2 is composed of a refraction section andnear-optical-axis optical surface area A3 is composed of a refractionsection, as shown in FIG. 25, or the constitution wherein outermostoptical surface area A1 is composed of a diffractive section,intermediate optical surface area A2 is composed of a mixture of adiffractive section and a refraction section and near-optical-axisoptical surface area A3 is composed of a refraction section, as shown inFIG. 26.

Though there is exemplified an objective lens wherein outside opticalsurface area A1 is composed of a diffractive section and inside opticalsurface area A2 is composed of a diffractive section as shown in FIG.15, in Examples 3-6, it is also possible to make the outside opticalsurface area A1 to be composed of a diffractive section and to make theinside optical surface area A2 to be composed of a refraction section asshown in FIG. 10. It is further possible to make the inside opticalsurface area A2 to be composed of a mixed existence of the diffractivesection and the refraction section.

Though an explanation of examples of these concrete structures will beomitted, they may easily be worked if the spirit of the invention isobserved. It is further possible to modify variously without departingfrom the spirit of the invention. For example, four or more opticalsurface areas may be used for composition as stated above, without beinglimited to the structure wherein functions can be divided by two opticalsurface areas or three optical surface areas.

Incidentally, the diffractive section may naturally be provided on thesurface of the corresponding area closer to a light source, or on thesurface of the corresponding area closer to an image, or even on bothsurfaces.

In the foregoing, “mixed existence” is not limited to the occasion wherea diffractive section and a refraction section are formed almosthalf-and-half as illustrated, and it can take various embodiments ofmixed existence.

Further, an embodiment of the optical pickup device is not limited tothe aforesaid embodiment, and for example, it can also be applied to atype of 2-light source and 2-optical detector.

The invention can naturally be applied not only to an optical pickupdevice capable of recording and/or reproducing of information for DVDand CD, but also to at least two optical information recording mediaeach having a different transparent base board thickness. In particular,it is especially beneficial to apply to optical information recordingmedia each having a different transparent base board thickness andhaving a different necessary numerical aperture. Further, for example,the invention can also be applied to an optical pickup device capable ofrecording and/or reproducing of information for only DVD, or it can beapplied as an objective lens to which a divergent light flux enters, oras an optical information recording medium employing that objectivelens.

Further, in the invention, with regard to a divergent light fluxentering an objective lens, it is not limited to the occasion wherein adivergent light flux emitted from a light source enters directly anobjective lens, and a coupling lens which changes an angle of divergenceof a divergent light flux emitted from a light source may be interposedbetween the light source and the objective lens, and what is essentialis that the divergent light flux can enter the objective lens.

The invention makes it possible to provide a practical objective lensand an optical pickup apparatus wherein a divergent light emitted from alight source enters the objective lens for a plurality of opticalinformation recording media each having a different transparent baseboard thickness, and sufficient capacity for changes of ambienttemperature used is satisfied while recording or reproducing of eachinformation is being made possible.

Embodiment of the Invention

The invention will further be explained in detail, referring to thedrawings as follows.

Seventh Embodiment

The seventh embodiment will be explained. FIG. 27 is a schematicstructure diagram of an optical pickup device including an objectivelens of the present embodiment. The optical pickup device is composed offirst light source 101 with wavelength λ₁ for DVD (first opticalinformation recording medium), second light source 102 with wavelengthλ₂ for CD (second optical information recording medium), beam splitter103 that makes a path for a light flux emitted from the light source 101to agree with that for a light flux emitted from the light source 102,objective lens 105 that converges each light flux, diaphragm 104 thatdetermines a diameter of a light flux incident on the objective lens105, an actuator (not shown) that drives the objective lens 105, and asensor (not shown) that detects a reflected light from opticalinformation recording medium ORM.

When recording or reproducing either one of DVD and CD, light-emittinglight source 101 or 102 is selected appropriately. Since a divergentlight flux enters the objective lens 105 and lateral magnification isfinite, aberration deterioration caused by temperature changes isworsened compared with an occasion wherein infinite light flux enters asstated above.

FIG. 28 is a sectional view of primary portions of objective lens 105.The objective lens 105 is composed of two-sided aspheric surfaces 105Aand 105B, and three optically functional surfaces 105 a, 105 b and 105 care formed on the surface 105A closer to the light source. The innermostoptically functional surface 105 a and outermost optically functionalsurface 105 c are represented by a refracting interface expressed by thesame aspherical coefficient. Intermediate optically functional surface105 b is a refracting interface expressed by aspherical coefficientwhich is different from that for adjoining optically functional surfaces105 a and 105 c on both sides, and aspherical aberration correction forthe intermediate optically functional surface is different from that foradjoining surfaces on both sides. Further, it is preferable thatrefractive index temperature dependency of a material (for example,glass) for the objective lens is lower, and the following expression issatisfactory.

|dn/dT|10.0×10⁻⁶(/° C.)  (2)

In that case, temperature characteristics are satisfactory even when adiffractive structure for improving temperature characteristics is notused. In this case, it is preferable that each of optically functionalsurfaces 105 a, 105 b and 105 c is formed to have a step at a boundarysection, and it is preferable that the step at the boundary section thatis farther from an optical axis is greater than that at the boundarysection that is closer to an optical axis, on the intermediate opticallyfunctional surface 105 b.

Now, a design for interchangeability for making it possible to record orreproduce for both DVD and CD will be explained. For light fluxespassing respectively through the inside and outside optically functionalareas 105 a and 105 c, it is possible to carry out spherical aberrationcorrection, assuming the use of DVD. However, with regard to lightfluxes passing respectively these optically functional surfaces 105 aand 105 c, over spherical aberration is generated because of adifference of a base board thickness when CD is used, which usuallymakes them to be unsuitable for recording or reproducing of CD.Therefore, intermediate optically functional surface 105 b isconstituted as follows.

FIG. 29 is a diagram showing an example of design (targetcharacteristics) for spherical aberration related to the presentembodiment. According to FIG. 29, a light flux passing through innermostoptically functional area 105 a is not aplanatic. However, when a lightflux diameter is stopped down at the position defocused from theparaxial image point by +10 μm, it is possible to secure the state wherethe residual aberration is smaller than Marechal criterion. However,since it is insufficient as a spot diameter formed on a recordingsurface of an optical information recording medium, there is formedintermediate optically functional area 105 b representing CD-exclusivearea where a spot diameter for CD is stopped down. To be concrete, it ispreferable to form intermediate optically functional area 105 b so thatlight-converging is made on the vicinity of the light spot formed on theoptical information recording medium at the aforesaid defocusedposition, and spherical aberration may be designed with assumedtransparent base board thickness t_(c) (t_(c)

(t₁+t₂)/2) which is between DVD transparent base board thickness t1 andCD transparent base board thickness t2.

When CD is used, a light flux passing through outside opticallyfunctional surface 105 c becomes a flare light to exist at the positionwhich is away by a distance that is about 10 times a size of a main spotdiameter. When DVD is used, a light flux passing through an intermediateoptically functional surface becomes a flare light to exist on anoutside zone which is away by a distance that is several times a size ofa main spot diameter. Therefore, if this flare light does not enter anunillustrated sensor element, or if the flare light is on the level thatis not problematic electrically for practical use, an aperture diametercan also be the same for both DVD and CD.

Further, for wavelength variation of light sources 101 and 102,objective lens 105 composed of a refracting interface is more stable,compared with an objective lens that is provided with a diffractivestructure which changes power depending on a wavelength. However,wavelength dependency of the refractive index is lowered as a dispersionvalue of glass material grows greater, which is preferable.

In this way, the objective lens 105 in the present embodiment canconduct recording or reproducing of information properly for both DVDand CD each having a different base board thickness, while correctingtemperature characteristics and wavelength characteristicsappropriately, even under the specifications which turn out to be morestrict for temperature characteristics.

Incidentally, the invention is not limited to the present embodiment.Namely, it is possible either to make the objective lens to be composedof cemented lenses or to make the surface of glass lens 105′ to becomposed of aspheric surface 105S made of UV-setting resin, as shown inFIG. 30. When the objective lens is made of different glass materials asstated above, at least the following expression needs to be satisfiedfor the glass material having stronger power (105′ in this case).

|dn/dT|≦10.0×10⁻⁶(/° C.)  (2)

When processing is taken into consideration, it is preferable to providethe aforesaid three optically functional surfaces 105 a, 105 b and 105 con the side of the surface 105 made of UV-setting resin. In this case,the objective lens can be applied also to the occasion where the samelight source wavelength is used for conducting recording and reproducingfor both DVD and CD. Even when three or more optically functionalsurfaces are used, the same effect can be attained sufficiently. It canfurther be applied to those wherein lateral magnification makestemperature characteristics to be mild, namely, the lateralmagnification is infinite. In some cases, there may be providedwavelength selecting diaphragm (restricting member) 104′ that restrictsa light flux passing through outside optically functional surface 105 cin the case of using CD, as shown in FIG. 31.

Eighth Embodiment

Next, the eighth embodiment will be explained. FIG. 32 is a sectionalview of primary portions related to the eighth embodiment. The presentembodiment is different from the first embodiment on the point that thediffractive structure is given to the objective lens so that it mayattain interchangeability, and explanation for the portions in thepresent embodiment overlapping with those in the first embodiment willbe omitted.

With regard to objective lens 205, diffractive structure 205D is formedon aspheric surface 205A closer to a light source to be solid with it asshown in FIG. 32 (a), among aspheric surfaces 205A and 205B on bothsides. This diffractive structure 205D is composed of two opticallyfunctional surfaces 205 a and 205 c which are different in terms ofdesign concept with a certain height that is close to the ray of lightstipulating numerical aperture NA in the case of using CD and serves asa boundary, as shown in FIG. 32 (b).

Namely, the inside optically functional surface 205 a has a diffractivestructure for correcting aberration for each transparent base boardthickness of DVD and CD, while the outside optically functional surface205 b has a diffractive structure that corrects aberration for atransparent base board thickness and creates a flare light for CD. FIG.33 is a diagram showing a design example (target characteristics) ofspherical aberration related to the present embodiment.

Even in the present embodiment, it is preferable that refractive indextemperature dependency of the glass material of the objective lens 205is low, and the following expression is preferable.

|dn/dT|≦10.0×10⁻⁶(/° C.)  (2)

If the range mentioned above is exceeded, it is necessary to enhanceeffectiveness of diffraction for temperature correction in thediffractive structure 205D, resulting in narrowed diffraction pitch anda decline of diffraction efficiency.

The invention is not limited to the present embodiment. Namely, it ispossible either to make the objective lens to be composed of cementedlenses or to make the surface of the objective lens to be composed ofaspheric surface 205S made of UV-setting resin, as shown in FIG. 34. Inthis case, it is preferable to provide the aforesaid two opticallyfunctional surfaces 205 a and 205 b on the surface of the UV-settingresin. The reason for the foregoing is as follows. It is necessary toincrease a depth of each diffraction for obtaining the same diffractioneffect, because a relative refractive index of materials becomes smallerwhen the diffractive structure is tried to be provided on the cementedportion. It is possible either to provide diffractive structures on bothsides of the objective lens 205, or to provide diffraction surfaces onthe plane where the diffractive section on the outside is different fromthat on the inside. Even when three or more optically functionalsurfaces are used for the structure, it is possible to form one havingthe same function. As shown in the example of the optical pickup devicein FIG. 35, it is also possible to provide coupling lens 206 between thesecond light source 102 and objective lens 205 to use it for the opticalinformation recording medium on the other side (CD in this case), takingdivergence angle characteristics of the second light source 102 intoconsideration. The objective lens can be applied also to an opticalsystem wherein lateral magnification of individual objective lens 205for DVD is not the same as that of individual objective lens 205 for CD.

Ninth Embodiment

Next, the ninth embodiment will be explained. In the present embodiment,a diffractive structure is formed on an objective lens, and design ofeach functional surface is different from that in the eighth embodiment,and explanation for the portions in the present embodiment overlappingwith those in the eighth embodiment will be omitted.

FIG. 36 is a sectional view of primary portions of the objective lens inthe present embodiment, and a value of refractive index temperaturecharacteristics dn/dT of the material for objective lens 305 isexpressed as follows.

|dn/dT|≦10.0×10⁻⁶(/° C.)  (2)

Both sides of the objective lens 305 are composed respectively ofrefracting interfaces 305A and 305B both representing an asphericsurface, and diffractive structure 305D is formed partially on an areaof surface 305A of the objective lens 305 closer to a light source. Inthis case, the objective lens 305 is composed of three opticallyfunctional surfaces 305 a, 305 b and 305 c, and further, a part of thearea in the vicinity of ray of light stipulating numerical aperture NAin the case of using CD is made to be of a diffractive structure, thusthe objective lens 305 is of the diffractive structure that makes theobjective lens 305 to be used for both of DVD and CD. Each of theoptically functional surfaces 305 a and 305 c on both sides is composedof a refracting interface to be an aspheric surface which is correctedin terms of spherical aberration mainly for DVD. Though the insideoptically functional surface 305 a is not designed for CD, it ispossible to stop down a spot diameter on the surface of an optical disceven for CD, when the inside optically functional surface 305 a isconnected together to spherical aberration on intermediate opticallyfunctional surface 305 b. FIG. 37 is a diagram showing an example ofdesign (target characteristics) for spherical aberration related to thepresent embodiment.

Incidentally, the invention is not limited to the present embodiment.Namely, it is possible either to make the objective lens 305 to becomposed of cemented lenses or to make the surface of a glass lens to becomposed of an aspheric surface made of UV-setting resin. In this case,it is preferable that the aforesaid three optically functional surfacesare provided on the surface side of the UV-setting resin.

Tenth Embodiment

Next, the fourth embodiment will be explained. In the presentembodiment, a diffractive structure is formed on an objective lens, anddesign of each functional surface is different from those in the eighthembodiment and the ninth embodiment, and explanation for the portions inthe present embodiment overlapping with those in each embodiment will beomitted.

FIG. 38 is a sectional view of primary portions of the objective lens inthe present embodiment, and a value of refractive index temperaturecharacteristics dn/dT of the material for the objective lens isexpressed as follows.

dn/dT|≦10.0×10⁻⁶(/° C.)  (2)

Both sides of the objective lens 405 are composed respectively ofrefracting interfaces 405A and 405B both representing an asphericsurface, and diffractive structure 405D is formed partially on an areaof surface 405A of the objective lens 405 closer to a light source. Inthis case, the objective lens 405 is composed of three opticallyfunctional surfaces 405 a, 405 b and 405 c, and further, a part of thearea in the vicinity of ray of light stipulating numerical aperture NAin the case of using CD is made to be of diffractive structure 405D thatmakes the objective lens 405 to be used for both of DVD and CD. Adiffraction surface is formed on outside optically functional surface405 c, spherical aberration is corrected on DVD, and a diffractivestructure which creates a flare is formed in CD. FIG. 39 is a diagramshowing a design example (target characteristics) of sphericalaberration related to the present embodiment.

Incidentally, the invention is not limited to the present embodiment.Namely, it is possible either to make the objective lens to be composedof cemented lenses or to make the surface of a glass lens to be composedof an aspheric surface made of UV-setting resin. In this case, it ispreferable that the aforesaid three optically functional surfaces areprovided on the surface side of the UV-setting resin.

Examples of the invention will be explained as follows.

Example 7

The present example is one for the objective lens related to the SeventhEmbodiment stated above. Table 8 shows lens data.

TABLE 8 Example 7 f1 = 3.00 mm, m1 = −1/7.0 NA1 = 0.60, NA2 = 0.45dn2/dT = +3.8 × E−6 (/° C.) at 632.8 nm, νd = 61.2 DVD CD ith ni nisurface ri di (650 nm) (650 nm) di (780 nm) (780 nm) 0 23.576 1.0 23.5761.0 Emission point 1 ∞ 0.0 1.0 0.0 1.0 Aperture φ4.06 mm 2 2.1759 2.21.58642 2.2 1.58252  2′ 2.1759 2.1962 1.58642 2.1962 1.58252  2″ 2.17592.2 1.58642 2.2 1.58252 3 −5.7537 1.928 1.0 1.566 1.0 4 ∞ 0.6 1.58 1.21.55 5 ∞ Aspherical data 2nd surface (0 < h < 1.32 mm: Inside opticallyfunctional surface) Aspherical coefficient κ −0.92846 × E−0 A1 −0.11050× E−2 P1 3.0 A2 +0.51090 × E−2 P2 4.0 A3 −0.16336 × E−2 P3 5.0 A4+0.57112 × E−3 P4 6.0 A5 +0.17007 × E−4 P5 8.0 A6 −0.73062 × E−5 P6 10.02′nd surface (1.32 mm < h < 1.54 mm: Intermediate optically functionalsurface) Aspherical coefficient κ −0.92421 × E−0 A1 −0.99146 × E−3 P13.0 A2 +0.51636 × E−2 P2 4.0 A3 −0.16069 × E−2 P3 5.0 A4 +0.58391 × E−3P4 6.0 A5 +0.19303 × E−4 P5 8.0 A6 −0.73840 × E−5 P6 10.0 2″nd surface(1.54 mm < h: Outside optical surface area) Aspherical coefficient κ−0.92846 × E−0 A1 −0.11050 × E−2 P1 3.0 A2 +0.51090 × E−2 P2 4.0 A3−0.16336 × E−2 P3 5.0 A4 +0.57112 × E−3 P4 6.0 A5 +0.17007 × E−4 P5 8.0A6 −0.73062 × E−5 P6 10.0 3rd surface Aspherical coefficient A1 +0.16009× E−1 P1 4.0 A2 −0.26764 × E−2 P2 6.0 A3 +0.30016 × E−3 P3 8.0 A4−0.17687 × E−4 P4 10.0

Each surface is composed of an aspheric surface, and each asphericsurface has an aspherical form expressed by “Numeral 4”.

Where, Z represents an axis along the optical axis direction, hrepresents a height perpendicular to the optical axis, r representsaxial curvature of radius, k represents the constant of the cone, Arepresents the aspherical coefficient and P represents the number ofpower of the aspheric surface. Further, three optically functionalsurfaces exist on the aspheric surface of the objective lens closer to alight source, and each of them is an aspheric surface expressed by“Numeral 4”.

Those to which the present example can be applied are simple opticalsystems wherein a divergent light flux emitted from each light source ofDVD and CD enters an objective lens directly. Glass materials for theobjective lens whose refractive index temperature dependency dn/dT is−5.8×10⁻⁶ (/° C.) were used. NA, temperature characteristics in the caseof using wavelength DVD and others are shown in Table 14. It is possibleto confirm that both temperature characteristics and wavelengthcharacteristics are improved, compared with a conventional example.

FIG. 40 represents a spherical aberration diagram of the present examplewherein three optically functional surfaces are formed. FIG. 41 showssimulation of PSF in the case of an occasion where a light flux withGaussian distribution enters the aforesaid objective lens by using afixed diaphragm that regulates a light flux corresponding to NA 0.60 onthe DVD side, and it shows a form of a spot on the information recordingsurface of the optical information recording medium. The aperturediameter in the case of CD is a result of simulation for the occasionwhere a light flux with the same aperture diameter as in DVD is made toenter. As is understood from this, a spot diameter (0.831×λ/NA (μm))requested on the recording surface is satisfied.

On the inside optically functional surface, residual sphericalaberration of about 0.02 λ₁ rms is generated on purpose for DVD. Thedesign of this kind makes it possible to reduce residual sphericalaberration in CD. In the present example, a light flux passing throughthe intermediate optically functional surface is corrected in terms ofspherical aberration for the optical information recording medium withassumed transparent base board thickness of t_(c)=1.0 mm, to be used forforming a spot in CD at a defocus position that is located on the overside by about 10 μm from a paraxial image point for CD.

As shown in Table 14, it is possible to realize an objective lens havinglateral magnification of m=− 1/7, NA of 0.60 and severe temperaturecharacteristics, wherein error characteristics are improved so that theobjective lens may by used for both DVD and CD.

Example 8

The present example is one related to the objective lens concerning theseventh embodiment stated above. Table 9 shows lens data.

TABLE 9 Example 8 f1 = 3.00 mm, m1 = −1/7.0 NA1 = 0.60, NA2 = 0.45dn2/dT = −1.2 × E−4 (/° C.) at 632.8 nm, νd = 57.0 dn3/dT = +0.8 × E−6(/° C.) at 632.8 nm, νd = 55.3 DVD CD ith ni ni surface ri di (650 nm)(650 nm) di (780 nm) (780 nm) 0 23.205 1.0 23.205 1.0 Emission point 1 ∞0.0 1.0 0.0 1.0 Aperture φ4.01 mm 2 2.600 0.1 1.48953 0.1 1.48616  2′2.600 0.0958 1.48953 0.0958 1.48616  2″ 2.600 0.1 1.48953 0.1 1.48616 32.1270 2.8 1.67447 2.8 1.66959 4 −6.0270 1.638 1.0 1.276 1.0 5 ∞ 0.61.58 1.2 1.55 6 ∞ Aspherical data 2nd surface (0 < h < 1.32 mm: Insideoptically functional surface) Aspherical coefficient κ −0.43271 × E+01A1 −0.26060 × E−2 P1 3.0 A2 +0.34891 × E−1 P2 4.0 A3 −0.65070 × E−2 P35.0 A4 −0.25906 × E−2 P4 6.0 A5 +0.57180 × E−3 P5 8.0 A6 −0.54866 × E−4P6 10.0 2′nd surface (1.32 mm < h < 1.51 mm: Intermediate opticallyfunctional surface) Aspherical coefficient κ −0.41771 × E+01 A1 −0.34857× E−2 P1 3.0 A2 +0.35107 × E−1 P2 4.0 A3 −0.64174 × E−2 P3 5.0 A4−0.25658 × E−2 P4 6.0 A5 +0.58143 × E−3 P5 8.0 A6 −0.57791 × E−4 P6 10.02″nd surface (1.51 mm < h: Outside optical surface area) Asphericalcoefficient κ −0.43271 × E+01 A1 −0.26060 × E−2 P1 3.0 A2 +0.34891 × E−1P2 4.0 A3 −0.65070 × E−2 P3 5.0 A4 −0.25906 × E−2 P4 6.0 A5 +0.57180 ×E−3 P5 8.0 A6 −0.54866 × E−4 P6 10.0 3rd surface Aspherical coefficientκ −0.16931 × E+01 A1 +0.47202 × E−2 P1 4.0

The objective lens in the present example is one wherein three opticallyfunctional surfaces (see FIG. 32) are formed with UV-setting resin onthe surface of one side of a glass lens. Refractive index temperaturedependency of the resin itself is −1.2×10⁻⁴ (/° C.) which is the same asthat in conventional example 2. However, it is possible to correcttemperature characteristics for the total objective lens, by reducingpower of the resin portion and by using one whose refractive indextemperature dependency of a glass lens on the other side is as small as+0.8×10⁻⁶ (/° C.). Since the design of interchangeability for DVD and CDis the same as in Example 1, the explanation therefore will be omitted.

FIG. 42 shows a spherical aberration diagram of the present example. Thespot form on the recording surface of each optical information recordingmedium is shown in FIG. 43.

As shown in Table 14, it is possible to realize an objective lens havinglateral magnification of m=− 1/7, NA of 0.60 and severe temperaturecharacteristics, wherein error characteristics are improved so that theobjective lens may by used for both DVD and CD.

Example 9

The present example is one related to the eighth embodiment statedabove. Table 10 shows lens data.

TABLE 10 Example 9 f1 = 3.00 mm, m1 = 0 NA1 = 0.65, NA2 = 0.45 dn2/dT =−5.7 × E−6 (/° C.) at 632.8 nm, νd = 81.6 DVD CD ith ni ni surface ri di(660 nm) (660 nm) di (790 nm) (790 nm) 0 ∞ 1.0 ∞ 1.0 Emission point 1 ∞0.0 1.0 0.0 1.0 Aperture φ3.90 mm 2 1.770 1.6 1.58642 1.6 1.58252 2′1.798 1.5999 1.58642 1.5999 1.58252 3 −6.422 1.725 1.0 1.353 1.0 4 ∞ 0.61.577 1.2 1.570 5 ∞ Aspherical data 2nd surface (0 < h < 1.37 mm: Insideoptically functional surface) Aspherical coefficient κ  −9.9350 × E−1 A1 +6.4273 × E−3 P1 4.0 A2  +6.2694 × E−4 P2 6.0 A3 −4.4974 × E−5 P3 8.0A4  +2.8692 × E−5 P4 10.0 A5  −2.5654 × E−5 P5 12.0 Optical pathdifference function (Coefficient of optical path difference function:Design basis wavelength 720 nm) B2  +2.4918 × E−4 B4  −2.0024 × E−3 B6 −3.7862 × E−4 B8  +2.0983 × E−4 B10  −5.8311 × E−5 2′nd surface (1.37mm < h: Outside optical surface area) Aspherical coefficient κ  −8.7077× E−1 A1  +6.2127 × E−3 P1 4.0 A2  +6.3107 × E−4 P2 6.0 A3  +1.3601 ×E−4 P3 8.0 A4  −2.5299 × E−5 P4 10.0 A5  −8.0092 × E−6 P5 12.0 Opticalpath difference function (Coefficient of optical path differencefunction: Design basis wavelength 660 nm) B2  −2.2736 × E−3 B4  −3.2476× E−4 B6  −8.8656 × E−5 B8  −1.5681 × E−5 B10  +5.2484 × E−6 3rd surfaceAspherical coefficient A1 +0.20368 × E−1 P1 4.0 A2 −0.48550 × E−2 P2 6.0A3 +0.72231 × E−3 P3 8.0 A4 −0.97114 × E−4 P4 10.0 A5 +0.78427 × E−5 P512.0 A6 −0.94305 × E−8 P6 14.0

Each of both sides of the objective lens of the present example is anaspheric surface, and a diffractive structure is provided solidly on thesurface of the aspheric surface on one side. As shown in FIG. 32, thisdiffractive structure is designed to be two different connected portionson both sides of the boundary represented by distance h from an opticalaxis. Namely, two optically functional surfaces are formed on thediffractive structure. The objective lens is made of glass materialwhose refractive index temperature dependency is −5.7×10⁻⁶ (/° C.).

For the light flux passing through the inside optically functionalsurface, there is provided a diffractive structure that correctsspherical aberration for a wavelength and a transparent base boardthickness used for DVD and for those used for CD. Further, on theoutside optically functional surface, there is provided a diffractivestructure that corrects spherical aberration for DVD, and generates overflare on purpose for CD.

In general, with respect to the diffractive structure, phase differencefunction ΦB is expressed by Numeral 1 with a unit of radian. By makingthe secondary coefficient to be a nonzero value, it is possible to giveparaxial power to the diffraction portion. In addition, by making thecoefficient of a phase difference function other than the secondarycoefficient such as, for example, fourth order coefficient or sixthorder coefficient to be a nonzero value, it is possible to controlspherical aberration. “Control” in this case means that the sphericalaberration of the refraction portion is corrected as a whole by givingspherical aberration that is opposite in terms of characteristic to theaforesaid spherical aberration to the diffraction portion, or totalspherical aberration is made to be a desired flare amount bymanipulating the spherical aberration of the diffraction portion. It istherefore possible to consider spherical aberration in temperaturechanges to be total of changes of spherical aberration of the refractionportion caused by temperature changes and spherical aberration changesof the diffraction portion.

With respect to changes caused by temperature in the refraction portion,an amount of changes is small because temperature dependency forrefractive index change of glass material is small. Therefore, it can besaid that temperature characteristics of the total objective lens turnout to be better, though spherical aberration caused by change ofspherical aberration of the diffraction portion. Small change ofspherical aberration of the diffraction portion in this case means is toweaken wavelength dependency, which results in that effectiveness ofdiffraction is weakened and a pitch of ring-shaped diffractive zone(diffraction pitch of the diffractive structure) is broadened.

With respect to the diffractive structure formed on the inside opticallyfunctional surface, a homogeneous diffracted light is used for DVD andCD, which is preferable compared with an occasion where anon-homogeneous diffracted light is used. In the present example, firstorder diffracted light is used for both DVD and CD. For the outsideoptically functional surface, a number of the order may either be theone which is the same as that for the inside optically functionalsurface, or be the one whose absolute value increases. Since the outsideoptically functional surface is not used usually for CD, it ispreferable that the standard wavelength (blazed wavelength) which makesthe diffraction efficiency to be highest on this functional surface ismade to be the wavelength that is close to DVD. If an absolute value ofthe number of the order for diffraction is made to be greater in thiscase, it is possible to lower the diffraction efficiency on the CD sideand thereby to lower CD flare, when the blazed wavelength is set in thevicinity of DVD. Incidentally, in the present example, the first orderwas used as a number of the order for also the outside opticallyfunctional surface, and with respect to the blazed wavelength, 720 nmwas used for the inside optically functional surface and 660 nm was usedfor the outside optically functional surface.

FIG. 44 is an aspheric surface diagram in the present example, and itsspot profile is shown in FIG. 45. Error characteristics are shown inTable 14. As shown in this table, it is understood that an objectivelens capable of being used for both DVD and CD which are improved interms of error characteristics can be realized. It is also understoodthat the minimum value of a pitch of the ring-shaped diffractive zone isgreater than that in Conventional example 3.

Example 10

The present example is also an example related to the eighth embodimentstated above. Table 11 shows lens data.

TABLE 11 Example 10 f1 = 3.00 mm, m1 = 0 NA1 = 0.65, NA2 = 0.50 dn2/dT =−1.2 × E−4 (/° C.) at 632.8 nm, νd = 56.0 dn3/dT = +7.4 × E−6 (/° C.) at632.8 nm, νd = 37.2 DVD CD ith ni ni surface ri di (660 nm) (660 nm) di(790 nm) (790 nm) 0 ∞ 1.0 ∞ 1.0 Emission point 1 ∞ 0.0 1.0 0.0 1.0Aperture φ3.90 mm 2 2.480 0.1 1.54076 0.1 1.53704  2′ 2.492 0.1011.54076 0.101 1.53704 3 2.505 2.0 1.82708 2.0 1.81900 4 −302.939 1.4911.0 1.136 1.0 5 ∞ 0.6 1.577 1.2 1.570 6 ∞ Aspherical data 2nd surface (0< h < 1.53 mm: Inside optically functional surface) Asphericalcoefficient κ  −9.4998 × E−1 A1  −2.1815 × E−4 P1 4.0 A2  −3.7775 × E−4P2 6.0 A3  −2.4169 × E−4 P3 8.0 A4  −7.3177 × E−6 P4 10.0 Optical pathdifference function (Coefficient of optical path difference function:Design basis wavelength 720 nm) B2  −4.2048 × E−4 B4  −3.8051 × E−4 B6 −4.0549 × E−4 B8  −3.1443 × E−5 B10  −1.1611 × E−5 2′nd surface (1.53mm < h: Outside optical surface area) Aspherical coefficient κ  −8.4719× E−1 A1  +6.6073 × E−4 P1 4.0 A2  −2.2175 × E−4 P2 6.0 A3  −3.0955 ×E−5 P3 8.0 A4  −4.4414 × E−7 P4 10.0 Optical path difference function(Coefficient of optical path difference function: Design basiswavelength 660 nm) B2  −5.0466 × E−4 B4  −1.3513 × E−5 B6  −2.3685 × E−5B8  −4.8511 × E−6 B10  +2.0574 × E−6 3rd surface Aspherical coefficientκ −0.90540 × E−2 A1 +0.16292 × E−4 P1 4.0 A2 −0.10622 × E−3 P2 6.0 A3−0.48106 × E−4 P3 8.0 A4 −0.90706 × E−5 P4 10.0 A5 −0.10113 × E−4 P512.0 A6 −0.41941 × E−5 P6 14.0 4th surface Aspherical coefficient κ+0.17083 × E+5 A1 +0.25872 × E−3 P1 4.0 A2 −0.44991 × E−4 P2 6.0 A3−0.69101 × E−4 P3 8.0 A4 −0.22469 × E−3 P4 10.0 A5 −0.58317 × E−4 P512.0 A6 +0.29543 × E−4 P6 14.0

The objective lens is one wherein two optically functional surfaces eachhaving a diffractive structure made of UV-setting resin are formed onthe surface on one side of a glass lens. Refractive index temperaturedependency of the resin itself is −1.2×10⁻⁴ (/° C.) which is the same asthat in conventional example 2. However, it is possible to correcttemperature characteristics of the total objective lens by weakeningpower of the resin portion and by using one wherein refractive indextemperature dependency of a glass lens on the other side is as small as+7.4×10⁻⁶ (/° C.).

Since the design for interchangeability of DVD and CD is the same asthat in Example 9, the explanation thereof will be omitted. FIG. 46shows a spherical aberration diagram of the present example. A form of aspot on a recording surface of each optical information recording mediumis shown in FIG. 47.

As shown in Table 14, it is understood that an objective lens capable ofbeing used for both DVD and CD improved in terms of errorcharacteristics can be realized in an objective lens wherein NA is 0.65and temperature characteristics are severe. It is also understood thatthe minimum value of a pitch of the ring-shaped diffractive zone isgreater than that in Conventional example 3.

Example 11

The present example is an example related to the eighth embodimentstated above. Table 12 shows lens data.

TABLE 12 Example 11 f1 = 3.00 mm, m1 = −1/7.0 NA1 = 0.60, NA2 = 0.45dn2/dT = −1.2 × E−4 (/° C.) at 632.8 nm, νd = 56.0 dn3/dT = +0.8 × E−6(/° C.) at 632.8 nm, νd = 55.3 DVD CD ith ni ni surface ri di (650 nm)(650 nm) di (780 nm) (780 nm) 0 26.225 1.0 26.225 1.0 Emission point 1 ∞0.0 1.0 0.0 1.0 Aperture φ4.0 mm 2 2.619 0.1 1.54112 0.1 1.53727  2′2.654 0.101 1.54112 0.101 1.53727 3 2.824 2.6 1.67424 2.0 1.66959 4−4.928 1.788 1.0 1.429 1.0 5 ∞ 0.6 1.577 1.2 1.570 6 ∞ Aspherical data2nd surface (0 < h < 1.584 mm: Inside optically functional surface)Aspherical coefficient κ  −4.6299 × E−0 A1  +2.0834 × E−2 P1 4.0 A2 −5.7851 × E−3 P2 6.0 A3  +9.6195 × E−4 P3 8.0 A4  −1.2123 × E−4 P4 10.0Optical path difference function (Coefficient of optical path differencefunction: Design basis wavelength 720 nm) B2  +7.9637 × E−4 B4  −1.4993× E−3 B6  −9.9900 × E−5 B8  +5.0721 × E−5 B10  −9.3677 × E−6 2′ndsurface (1.584 mm < h: Outside optical surface area) Asphericalcoefficient κ  −4.8750 × E−0 A1  +2.2234 × E−2 P1 4.0 A2  −5.7025 × E−3P2 6.0 A3  +9.4382 × E−4 P3 8.0 A4  −1.2143 × E−4 P4 10.0 Optical pathdifference function (Coefficient of optical path difference function:Design basis wavelength 650 nm) B2  −9.4134 × E−4 B4  −2.4877 × E−4 B6 −8.0210 × E−5 B8  −1.3836 × E−5 B10  +2.0287 × E−6 3rd surfaceAspherical coefficient κ −0.25997 × E−0 A1 −0.31934 × E−2 P1 4.0 A2−0.60892 × E−3 P2 6.0 A3 −0.10705 × E−3 P3 8.0 A4 −0.55001 × E−4 P4 10.04th surface Aspherical coefficient κ +0.15272 × E+0 A1 +0.84547 × E−2 P14.0 A2 −0.32078 × E−2 P2 6.0 A3 +0.16251 × E−3 P3 8.0 A4 +0.10235 × E−4P4 10.0 A5 +0.30261 × E−5 P5 12.0 A6 −0.64029 × E−6 P6 14.0

This is an example wherein a divergent light flux enters an objectivelens. The objective lens is one wherein two optically functionalsurfaces each having a diffractive structure made of UV-setting resinare formed on the surface on one side of a glass lens. Refractive indextemperature dependency of the resin itself is −1.2×10⁻⁴ (/° C.) which isthe same as that in conventional example 2. However, it is possible tocorrect temperature characteristics of the total objective lens byweakening power of the resin portion and by using one wherein refractiveindex temperature dependency of a glass lens on the other side is assmall as +0.8×10⁻⁶ (/° C.).

Since an idea for forming two optically functional surfaces by providinga diffractive structure and a concept of design for aberration are thesame as those in Example 9, explanation therefore will be omitted. FIG.48 is a spherical aberration diagram of the present example, and a formof a spot on a recording surface of each optical information recordingmedium is shown in FIG. 49.

Table 14 shows error characteristics. As shown in this table, it isunderstood that an objective lens capable of being used for both DVD andCD improved in terms of error characteristics can be realized in anobjective lens with specifications wherein lateral magnification m1 is −1/7 and NA is 0.65 and temperature characteristics are severe. It isalso understood that the minimum value of a pitch of the ring-shapeddiffractive zone is greater than that in Conventional example 3.

Example 12

The present example is an example related to the eighth embodimentstated above. Table 13 shows lens data.

TABLE 13 Example 12 f1 = 3.00 mm, m1 = −1/10.0 NA1 = 0.60, NA2 = 0.45dn2/dT = −5.8 × E−6 (/° C.) at 632.8 nm, νd = 81.6 DVD CD ith ni nisurface ri di (650 nm) (650 nm) di (780 nm) (780 nm) 0 32.5 1.0 32.5 1.0Emission point 1 ∞ 0.0 1.0 0.0 1.0 Aperture φ3.91 mm 2 2.001 2.2 1.495292.2 1.49282  2′ 1.959 2.205 1.49529 2.205 1.49282 3 −4.141 1.776 1.01.381 1.0 4 ∞ 0.6 1.577 1.2 1.570 5 ∞ Aspherical data 2nd surface (0 < h< 1.37 mm: Inside optically functional surface) Aspherical coefficient κ −1.1326 × E−0 A1  +3.273 × E−3 P1 4.0 A2  +6.2694 × E−4 P2 6.0 A3 −4.4974 × E−5 P3 8.0 A4  +2.8692 × E−5 P4 10.0 A5  −2.5654 × E−5 P512.0 Optical path difference function (Coefficient of optical pathdifference function: Design basis wavelength 720 nm) B2  +2.4918 × E−4B4  −2.0024 × E−3 B6  −3.7862 × E−4 B8  +2.0983 × E−4 B10  −5.8311 × E−52′nd surface (1.37 mm < h: Outside optical surface area) Asphericalcoefficient κ  −8.7077 × E−1 A1  +6.2127 × E−3 P1 4.0 A2  +6.3107 × E−4P2 6.0 A3  +1.3601 × E−4 P3 8.0 A4  −2.5299 × E−5 P4 10.0 A5  −8.0092 ×E−6 P5 12.0 Optical path difference function (Coefficient of opticalpath difference function: Design basis wavelength 660 nm) B2  −2.2736 ×E−3 B4  −3.2476 × E−4 B6  −8.8656 × E−5 B8  −1.5681 × E−5 B10  +5.2484 ×E−6 3rd surface Aspherical coefficient A1 +0.20368 × E−1 P1 4.0 A2−0.48550 × E−2 P2 6.0 A3 +0.72231 × E−3 P3 8.0 A4 −0.97114 × E−4 P4 10.0A5 +0.78427 × E−5 P5 12.0 A6 −0.94305 × E−8 P6 14.0

This is an example wherein a divergent light flux enters an objectivelens. The objective lens wherein refractive index temperature dependencyis −5.8×10⁻⁶ (/° C.) was used. Each of both sides of the objective lensis an aspheric surface, and a diffractive structure is provided solidlyon the surface of the aspheric surface on one side as shown in FIG. 32,and two optionally functional surfaces are arranged thereon. Since thedesign of aberration is the same as that in Example 3, it will beomitted. FIG. 50 is a spherical aberration diagram of the presentexample, and a form of a spot on a recording surface of each opticalinformation recording medium is shown in FIG. 51.

Table 14 shows error characteristics. As shown in this table, it isunderstood that an objective lens capable of being used for both DVD andCD improved in terms of error characteristics can be realized in anobjective lens wherein lateral magnification m1 is − 1/7 and NA is 0.60.It is also understood that the minimum value of a pitch of thering-shaped diffractive zone is greater than that in Conventionalexample 3.

In addition to the examples described above, it is also possible toconstitute as follows. For example, an intermediate optically functionalsurface is made to be of a diffractive structure as illustrated in theninth embodiment, and both sides of the intermediate opticallyfunctional surface are constituted with a refracting interface as shownin the seventh embodiment. In this case, the diffractive structurecorrects spherical aberration of DVD, and it may be one which gives thesame spherical aberration as in CD of the First embodiment, for CD. FIG.36 shows a schematic sectional view of a lens, and FIG. 37 shows anexample of spherical aberration.

It is further possible to provide a diffractive structure on the outsideoptically functional surface as mentioned in the tenth embodiment. Inthis case, correction of spherical aberration in DVD and control offlare amount in CD are possible. FIG. 38 shows a schematic sectionalview of a lens, and FIG. 39 shows an example of spherical aberration.

Furthermore, it is naturally possible to improve focus characteristicson the CD side by providing a diaphragm with a structure that lowers atransmission factor or blocks for a light flux passing through theoutside optically functional surface in the case of CD, or anantireflection coating.

The invention makes it possible to provide an objective lens and anoptical pickup device wherein recording and reproducing for opticalinformation recording media each having a different transparent baseboard thickness are made possible, by forming different opticallyfunctional surfaces on the objective lens while keeping temperaturecharacteristics in the objective lens having specifications which maketemperature characteristics to be strict.

1-308. (canceled)
 309. An objective lens for use in an optical pickupdevice having a first light source to emit a light flux having awavelength λ1; a second light source to emit a light flux having awavelength λ2 (λ1λ2); and a light converging optical system; wherein theoptical pickup apparatus conducts recording or reproducing informationfor a first optical information recording medium including transparentbase board having a thickness t₁ by using the first light source and thelight converging optical system, and the optical pickup apparatusconducts recording or reproducing information for a second opticalinformation recording medium including a transparent base board having athickness t₂ (t₁<t₂) by using the second light source and the lightconverging optical system, the object lens comprising: at least twokinds of optically functional surfaces which regionrranged in adirection perpendicular to an optical axis and have a different opticalaction from others; wherein at least an outermost optically functionalsurface among the optically functional surfaces is used only whenrecording or reproducing information for the first optical informationrecording medium, wherein the objective lens is a pasted lens in whichat least two kinds of optical elements each made of a different opticalmaterial from other, and wherein a value of refractive index changedn/dT for a temperature change of an optical material used for anoptical element having a power component stronger than that of othersatisfies the following conditional formula under the conditions of thewavelength of the light source and a room temperature environment.|dn/dT|≦10.0×10⁻⁶(/° C.)
 310. The objective lens of claim 309, whereinat least one of optical elements other than those having stronger powercomponents among the aforesaid plural optical elements is made ofplastic material. 311-312. (canceled)
 313. The objective lens of claim309, wherein the objective lens is composed only of a refractinginterface, at least three optically functional surfaces are formed, alight flux passing through the innermost optically functional surface isused for conducting recording or reproducing of information for thefirst and second optical information recording media, a light fluxpassing through the intermediate optically functional surface is usedfor conducting recording or reproducing of information for the secondoptical information recording medium, and a light flux passing throughthe outermost optically functional surface is used for conductingrecording or reproducing of information for the first opticalinformation recording medium. 314-315. (canceled)
 316. The objectivelens of claim 309, wherein the objective lens has at least one of themhas a diffractive structure among the optically functional surfaces, anda optically functional surface closest to the optical axis has afunction to correct a spherical aberration when recording or reproducinginformation for the first and second optical information recordingmedia, and an outermost optically functional surface has a function tocorrect a spherical aberration when recording or reproducing informationfor the first optical information recording media and to make an overspherical aberration when recording or reproducing information for thesecond optical information recording media. 317-325. (canceled)
 326. Theobjective lens of claim 309, wherein image forming magnification m1 ofthe objective lens for conducting recording or reproducing ofinformation for the first optical information recording medium satisfiesthe following conditional formula.−¼≦m1≦⅛
 327. (canceled)
 328. The objective lens of claim 309, wherein anaperture-stop in the case of conducting recording or reproducing ofinformation for the first optical information recording medium is thesame as that in the case of conducting recording or reproducing ofinformation for the second optical information recording medium. 329.The objective lens of claim 309, wherein necessary numerical apertureNA1 in the case of conducting recording or reproducing of informationfor the first optical information recording medium satisfies thefollowing conditional formula.NA1≧0.60
 330. The objective lens of claim 309, wherein wavelength λ1 ofthe first light source is not more than 670 nm.
 331. The objective lensof claim 309, wherein the optical material is an optical glass anddispersion value vd is greater than
 50. 332. An optical pickup device,comprising: a first light source to emit a light flux having awavelength λ1; a second light source to emit a light flux having awavelength λ2 (λ1<λ1); and a light converging optical system; whereinthe optical pickup apparatus conducts recording or reproducinginformation for a first optical information recording medium includingtransparent base board having a thickness t₁ by using the first lightsource and the light converging optical system, and the optical pickupapparatus conducts recording or reproducing information for a secondoptical information recording medium including a transparent base boardhaving a thickness t₂ (t₁<t₂) by using the second light source and thelight converging optical system, wherein the object lens comprises atleast two kinds of optically functional surfaces which regionrranged ina direction perpendicular to an optical axis and have a differentoptical action from others; wherein at least an outermost opticallyfunctional surface among the optically functional surfaces is used onlywhen recording or reproducing information for the first opticalinformation recording medium, wherein the objective lens is a pastedlens in which at least two kinds of optical elements each made of adifferent optical material from other, and wherein a value of refractiveindex change dn/dT for a temperature change of an optical material usedfor an optical element having a power component stronger than that ofother satisfies the following conditional formula under the conditionsof the wavelength of the light source and a room temperatureenvironment.|dn/dT|≦100.0×10⁻⁶(/° C.)
 333. The optical pickup apparatus of claim332, wherein at least one of optical elements other than the opticalelement having stronger power component among the aforesaid pluraloptical elements is made of a plastic material.
 334. (canceled)
 335. Theoptical pickup apparatus of claim 332, wherein each optically functionalsurface mentioned above is formed to have a step at the boundarysection.
 336. The optical pickup apparatus of claim 332, wherein theobjective lens is composed only of a refracting interface, at leastthree optically functional surfaces are formed, a light flux passingthrough the innermost optically functional surface is used forconducting recording or reproducing of information for the first andsecond optical information recording media, a light flux passing throughthe intermediate optically functional surface is used for conductingrecording or reproducing of information for the second opticalinformation recording medium, and a light flux passing through theoutermost optically functional surface is used for conducting recordingor reproducing of information for the first optical informationrecording medium.
 337. (canceled)
 338. The optical pickup apparatus ofclaim 332, wherein the innermost optically functional surface and theoutermost optically functional surface have a function to correct anspherical aberration 0.04 λ₁rms or less when recording or reproducing ofinformation for the first optical information recording medium and theintermediate optically functional surface has a function to correct aspherical aberration to be smallest for an optical information recordingmedium having a thickness tc (t1<tc<t2).
 339. The optical pickupapparatus of claim 332, wherein the objective lens has at least one ofthem has a diffractive structure among the optically functionalsurfaces, and a optically functional surface closest to the optical axishas a function to correct a spherical aberration when recording orreproducing information for the first and second optical informationrecording media, and an outermost optically functional surface has afunction to correct a spherical aberration when recording or reproducinginformation for the first optical information recording media and tomake an over spherical aberration when recording or reproducinginformation for the second optical information recording media. 340-348.(canceled)
 349. The optical pickup apparatus of claim 332, wherein imageforming magnification m1 of the objective lens for conducting recordingor reproducing of information for the first optical informationrecording medium satisfies the following conditional formula.−¼≦m1≦⅛
 350. (canceled)
 351. The optical pickup apparatus of claim 332,wherein an aperture-stop in the case of conducting recording orreproducing of information for the first optical information recordingmedium is the same as that in the case of conducting recording orreproducing of information for the second optical information recordingmedium.
 352. The optical pickup apparatus of claim 332, whereinnecessary numerical aperture NA1 in the case of conducting recording orreproducing of information for the first optical information recordingmedium satisfies the following conditional formula.NA1≧0.60
 353. The optical pickup apparatus of claim 332, whereinwavelength λ₁ of the first light source is not more than 670 nm. 354.The optical pickup apparatus of claim 332, wherein the optical materialis an optical glass and dispersion value vd is greater than
 50. 355-358.(canceled)